Fix a segfault in Admin for empty queries

lib/ProxySQL_Admin.cpp

@@ -2320,10 +2320,12 @@ void admin_session_handler(MySQL_Session *sess, void *_pa, PtrSize_t *pkt) {
 	unsigned int query_no_space_length=remove_spaces(query_no_space);
 	//fprintf(stderr,"%s----\n",query_no_space);
 
-	// fix bug #925
-	while (query_no_space[query_no_space_length-1]==';' || query_no_space[query_no_space_length-1]==' ') {
-		query_no_space_length--;
-		query_no_space[query_no_space_length]=0;
+	if (query_no_space_length) {
+		// fix bug #925
+		while (query_no_space[query_no_space_length-1]==';' || query_no_space[query_no_space_length-1]==' ') {
+			query_no_space_length--;
+			query_no_space[query_no_space_length]=0;
+		}
 	}
 
 	// add global mutex, see bug #1188