MDEV-24827: Follow-up patch to fix compilation warning

Mixed declarations and code is not allowed for C90 so
fix it to avoid compilation break on some platforms.

sql/sql_select.cc

@@ -12960,6 +12960,12 @@ return_zero_rows(JOIN *join, select_result *result, List<TABLE_LIST> &tables,
     if (!send_error)
       result->send_eof();				// Should be safe
   }
+  /*
+    JOIN::join_free() must be called after the virtual method
+    select::send_result_set_metadata() returned control since
+    implementation of this method could use data strutcures
+    that are released by the method JOIN::join_free().
+  */
   join->join_free();
 
   DBUG_RETURN(0);

tests/mysql_client_test.c

@@ -19967,6 +19967,9 @@ static void test_mdev24827()
   int rc;
   MYSQL_STMT *stmt;
   unsigned long cursor = CURSOR_TYPE_READ_ONLY;
+  const char* query=
+    "SELECT t2.c1 AS c1 FROM t1 LEFT JOIN t2 ON t1.c1 = t2.c1 "
+    "WHERE EXISTS (SELECT 1 FROM t1 WHERE c2 = -1) ORDER BY c1";
 
   myheader("test_mdev24827");
 
@@ -19991,10 +19994,6 @@ static void test_mdev24827()
                   "SELECT seq, seq FROM seq_1_to_20000");
   myquery(rc);
 
-  const char* query=
-      "SELECT t2.c1 AS c1 FROM t1 LEFT JOIN t2 ON t1.c1 = t2.c1 "
-      "WHERE EXISTS (SELECT 1 FROM t1 WHERE c2 = -1) ORDER BY c1";
-
   stmt= mysql_stmt_init(mysql);
   check_stmt(stmt);