centralize disposal of parsed IMAP lists

makes the code less cluttered, and it's harder to introduce leaks.

this has the hypothetical disadvantage that due to freeing being
delayed, the peak memory usage would rise significantly if we chained to
another parse_list() call which produces a big list while already
holding a big list, but that isn't the case anywhere.

src/drv_imap.c

@@ -886,6 +886,7 @@ parse_list_continue( imap_store_t *ctx, char *s )
 		list = (resp == LIST_BAD) ? NULL : ctx->parse_list_sts.head;
 		ctx->parse_list_sts.head = NULL;
 		resp = ctx->parse_list_sts.callback( ctx, list, s );
+		free_list( list );
 	}
 	return resp;
 }
@@ -910,7 +911,6 @@ parse_namespace_rsp( imap_store_t *ctx, list_t *list, char *s )
 	if (!list) {
 	  bad:
 		error( "IMAP error: malformed NAMESPACE response\n" );
-		free_list( list );
 		return LIST_BAD;
 	}
 	if (list->val != NIL) {
@@ -931,22 +931,19 @@ parse_namespace_rsp( imap_store_t *ctx, list_t *list, char *s )
 			ctx->ns_delimiter = nsp_1st_dl->val[0];
 		// Namespace response extensions may follow here; we don't care.
 	}
-	free_list( list );
 
 	return parse_list( ctx, s, parse_namespace_rsp_p2 );
 }
 
 static int
-parse_namespace_rsp_p2( imap_store_t *ctx, list_t *list, char *s )
+parse_namespace_rsp_p2( imap_store_t *ctx, list_t *list ATTR_UNUSED, char *s )
 {
-	free_list( list );
 	return parse_list( ctx, s, parse_namespace_rsp_p3 );
 }
 
 static int
-parse_namespace_rsp_p3( imap_store_t *ctx ATTR_UNUSED, list_t *list, char *s ATTR_UNUSED )
+parse_namespace_rsp_p3( imap_store_t *ctx ATTR_UNUSED, list_t *list ATTR_UNUSED, char *s ATTR_UNUSED )
 {
-	free_list( list );
 	return LIST_OK;
 }
 
@@ -982,7 +979,6 @@ parse_fetch_rsp( imap_store_t *ctx, list_t *list, char *s ATTR_UNUSED )
 
 	if (!is_list( list )) {
 		error( "IMAP error: bogus FETCH response\n" );
-		free_list( list );
 		return LIST_BAD;
 	}
 
@@ -1105,7 +1101,6 @@ parse_fetch_rsp( imap_store_t *ctx, list_t *list, char *s ATTR_UNUSED )
 			if (cmdp->param.uid == uid)
 				goto gotuid;
 		error( "IMAP error: unexpected FETCH response (UID %u)\n", uid );
-		free_list( list );
 		return LIST_BAD;
 	  gotuid:
 		msgdata = ((imap_cmd_fetch_msg_t *)cmdp)->msg_data;
@@ -1131,7 +1126,6 @@ parse_fetch_rsp( imap_store_t *ctx, list_t *list, char *s ATTR_UNUSED )
 			cur->gen.tuid[0] = 0;
 	}
 
-	free_list( list );
 	return LIST_OK;
 }
 
@@ -1239,16 +1233,12 @@ parse_list_rsp( imap_store_t *ctx, list_t *list, char *cmd )
 	list_t *lp;
 
 	if (!is_list( list )) {
-		free_list( list );
 		error( "IMAP error: malformed LIST response\n" );
 		return LIST_BAD;
 	}
 	for (lp = list->child; lp; lp = lp->next)
-		if (is_atom( lp ) && !strcasecmp( lp->val, "\\NoSelect" )) {
-			free_list( list );
+		if (is_atom( lp ) && !strcasecmp( lp->val, "\\NoSelect" ))
 			return LIST_OK;
-		}
-	free_list( list );
 	return parse_list( ctx, cmd, parse_list_rsp_p1 );
 }
 
@@ -1257,7 +1247,6 @@ parse_list_rsp_p1( imap_store_t *ctx, list_t *list, char *cmd ATTR_UNUSED )
 {
 	if (!is_opt_atom( list )) {
 		error( "IMAP error: malformed LIST response\n" );
-		free_list( list );
 		return LIST_BAD;
 	}
 	if (!ctx->delimiter[0] && is_atom( list ))
@@ -1297,7 +1286,6 @@ parse_list_rsp_p2( imap_store_t *ctx, list_t *list, char *cmd ATTR_UNUSED )
 
 	if (!is_atom( list )) {
 		error( "IMAP error: malformed LIST response\n" );
-		free_list( list );
 		return LIST_BAD;
 	}
 	arg = list->val;
@@ -1308,7 +1296,7 @@ parse_list_rsp_p2( imap_store_t *ctx, list_t *list, char *cmd ATTR_UNUSED )
 		memcpy( arg, "INBOX", 5 );
 	} else if ((l = strlen( ctx->prefix ))) {
 		if (!starts_with( arg, argl, ctx->prefix, l ))
-			goto skip;
+			return LIST_OK;
 		arg += l;
 		argl -= l;
 		// A folder named "INBOX" would be indistinguishable from the
@@ -1318,19 +1306,17 @@ parse_list_rsp_p2( imap_store_t *ctx, list_t *list, char *cmd ATTR_UNUSED )
 		if (is_INBOX( ctx, arg, argl )) {
 			if (!arg[5])  // No need to complain about subfolders as well.
 				warn( "IMAP warning: ignoring INBOX in %s\n", ctx->prefix );
-			goto skip;
+			return LIST_OK;
 		}
 	}
 	if (argl >= 5 && !memcmp( arg + argl - 5, ".lock", 5 )) /* workaround broken servers */
-		goto skip;
+		return LIST_OK;
 	if (map_name( arg, (char **)&narg, offsetof(string_list_t, string), ctx->delimiter, "/") < 0) {
 		warn( "IMAP warning: ignoring mailbox %s (reserved character '/' in name)\n", arg );
-		goto skip;
+		return LIST_OK;
 	}
 	narg->next = ctx->boxes;
 	ctx->boxes = narg;
-  skip:
-	free_list( list );
 	return LIST_OK;
 }