Further improvement on PR #1922

lib/mysql_connection.cpp

@@ -1647,20 +1647,25 @@ bool MySQL_Connection::IsKeepMultiplexEnabledVariables(char *query_digest_text)
 		query_digest_text_filter_select=(char*)malloc(query_digest_text_len-7+1);
 		memcpy(query_digest_text_filter_select,&query_digest_text[7],query_digest_text_len-7);
 		query_digest_text_filter_select[query_digest_text_len-7]='\0';
+	} else {
+		return false;
 	}
 	//filter @@session. and @@
 	char *match=NULL;
-	while ((match = strcasestr(query_digest_text_filter_select,"@@session."))) {
+	while (query_digest_text_filter_select && (match = strcasestr(query_digest_text_filter_select,"@@session."))) {
 		*match = '\0';
 		strcat(query_digest_text_filter_select, match+strlen("@@session."));
 	}
-	while ((match = strcasestr(query_digest_text_filter_select,"@@"))) {
+	while (query_digest_text_filter_select && (match = strcasestr(query_digest_text_filter_select,"@@"))) {
 		*match = '\0';
 		strcat(query_digest_text_filter_select, match+strlen("@@"));
 	}
 
 	std::vector<char*>query_digest_text_filter_select_v;
-	char* query_digest_text_filter_select_tok=strtok(query_digest_text_filter_select, ",");
+	char* query_digest_text_filter_select_tok = NULL;
+	if (query_digest_text_filter_select) {
+	query_digest_text_filter_select_tok = strtok(query_digest_text_filter_select, ",");
+	}
 	while(query_digest_text_filter_select_tok){
 		//filter "as"/space/alias,such as select @@version as a, @@version b
 		while (1){