"Fixed invalid reads due to memory from 'first_comment' being freed from global 'stmt_info' #3427"

'CurrentQuery.QueryParserArgs.first_comment' needs to hold it's own copy
of 'first_comment', otherwise, the 'first_comment' from global 'stmt_info'
will be freed by 'Query_Info::end' at the end of the life of 'CurrentQuery'.

lib/MySQL_Session.cpp

@@ -4440,12 +4440,10 @@ handler_again:
 								stmt_info=GloMyStmt->find_prepared_statement_by_stmt_id(CurrentQuery.stmt_global_id);
 								CurrentQuery.QueryLength=stmt_info->query_length;
 								CurrentQuery.QueryPointer=(unsigned char *)stmt_info->query;
-								// NOTE: Since 'first_comment' is part of the info used to
-								// compute 'stmt' hashes, it's required to be copied here.
-								// Not doing so will lead to a loop of 'PREPARE' and 'EXECUTE',
-								// since the correct prepared statement will never be found
-								// due to the missing information to compute the hash.
-								CurrentQuery.QueryParserArgs.first_comment=stmt_info->first_comment;
+								// NOTE: Update 'first_comment' with the the from the retrieved
+								// 'stmt_info' from the found prepared statement. 'CurrentQuery' requires its
+								// own copy of 'first_comment' because it will later be free by 'QueryInfo::end'.
+								CurrentQuery.QueryParserArgs.first_comment=strdup(stmt_info->first_comment);
 								previous_status.push(PROCESSING_STMT_EXECUTE);
 								NEXT_IMMEDIATE(PROCESSING_STMT_PREPARE);
 								if (CurrentQuery.stmt_global_id!=stmt_info->statement_id) {