MDEV-4516 SELECT from I_S.QUERY_CACHE_INFO produces ER_UNKNOWN_ERROR when query cache size is 0

if qc->try_lock() fails, it's not an error

mysql-test/suite/plugins/r/qc_info.result

@@ -12,4 +12,7 @@ test	select * from t1	1	512
 drop table t1;
 select statement_schema, statement_text, result_blocks_count, result_blocks_size from information_schema.query_cache_info;
 statement_schema	statement_text	result_blocks_count	result_blocks_size
+set global query_cache_size = 0;
+select * from information_schema.query_cache_info;
+STATEMENT_SCHEMA	STATEMENT_TEXT	RESULT_BLOCKS_COUNT	RESULT_BLOCKS_SIZE	RESULT_BLOCKS_SIZE_USED
 set global query_cache_size= default;

mysql-test/suite/plugins/t/qc_info.test

@@ -4,5 +4,8 @@ drop table t1;
 # the query was invalidated
 select statement_schema, statement_text, result_blocks_count, result_blocks_size from information_schema.query_cache_info;
 
+set global query_cache_size = 0;
+select * from information_schema.query_cache_info;
+
 set global query_cache_size= default;
 

plugin/qc_info/qc_info.cc

@@ -83,7 +83,7 @@ static int qc_info_fill_table(THD *thd, TABLE_LIST *tables,
     return 0;
 
   if (qc->try_lock(thd))
-    return status;
+    return 0; // QC is or is being disabled
 
   /* loop through all queries in the query cache */
   for (uint i= 0; i < queries->records; i++)