Work around useless SASL warnings

Ater sasl_client_step() is called and the Cyrus SASL library forwards
it to the client plugin, if the result value is OK (authentication
succeeded), the clientout is filled out to be an empty string, even if
the client plugin wanted to return NULL.

To avoid that mbsync complains at this point, check the returned length
instead of the pointer.

src/drv_imap.c

@@ -2073,7 +2073,7 @@ done_sasl_auth( imap_store_t *ctx, imap_cmd_t *cmd ATTR_UNUSED, int response )
 		int rc = sasl_client_step( ctx->sasl, NULL, 0, &interact, &out, &out_len );
 		if (process_sasl_step( ctx, rc, NULL, 0, interact, &out, &out_len ) < 0)
 			warn( "Warning: SASL reported failure despite successful IMAP authentication. Ignoring...\n" );
-		else if (out)
+		else if (out_len > 0)
 			warn( "Warning: SASL wants more steps despite successful IMAP authentication. Ignoring...\n" );
 	}