MDEV-11857 json_search() shows "Out of memory" with empty key.

        We should be ready for an empty key.

mysql-test/t/func_json.test

@@ -220,3 +220,8 @@ SELECT json_replace('1', '$[0][0]', 100);
 SELECT json_replace('[]', '$[0][0]', 100);
 SELECT json_set('[]', '$[0][0]', 100);
 SELECT json_set('[]', '$[0][0][0]', 100);
+
+#
+# MDEV-11857 json_search() shows "Out of memory" with empty key.
+#
+SELECT JSON_search( '{"": "a"}', "one", 'a');

sql/item_jsonfunc.cc

@@ -147,8 +147,10 @@ static int json_nice(json_engine_t *je, String *nice_js,
         const uchar *key_start= je->s.c_str;
         const uchar *key_end;
 
-        while (json_read_keyname_chr(je) == 0)
+        do
+        {
           key_end= je->s.c_str;
+        } while (json_read_keyname_chr(je) == 0);
         
         if (je->s.error)
           goto error;
@@ -892,8 +894,10 @@ static int check_contains(json_engine_t *js, json_engine_t *value)
 
       DBUG_ASSERT(value->state == JST_KEY);
       k_start= value->s.c_str;
-      while (json_read_keyname_chr(value) == 0)
+      do
+      {
         k_end= value->s.c_str;
+      } while (json_read_keyname_chr(value) == 0);
 
       if (value->s.error || json_read_value(value))
         return FALSE;
@@ -2527,10 +2531,10 @@ String *Item_func_json_keys::val_str(String *str)
     {
     case JST_KEY:
       key_start= je.s.c_str;
-      while (json_read_keyname_chr(&je) == 0)
+      do
       {
         key_end= je.s.c_str;
-      }
+      } while (json_read_keyname_chr(&je) == 0);
       if (je.s.error ||
           (n_keys > 0 && str->append(", ", 2)) ||
           str->append("\"", 1) ||

strings/json_lib.c

@@ -1701,8 +1701,10 @@ int json_get_path_next(json_engine_t *je, json_path_t *p)
     {
     case JST_KEY:
       p->last_step->key= je->s.c_str;
-      while (json_read_keyname_chr(je) == 0)
+      do
+      {
         p->last_step->key_end= je->s.c_str;
+      } while (json_read_keyname_chr(je) == 0);
       if (je->s.error)
         return 1;
       /* Now we have je.state == JST_VALUE, so let's handle it. */