MDEV-11489 Assertion `0' failed in json_find_path.

When the json was just a scalar value, json_extract tried
        to parse after the value ended.

mysql-test/r/func_json.result

@@ -138,6 +138,9 @@ json_extract('[10, 20, [30, 40]]', '$[2][*]')
 select json_extract('[10, 20, [{"a":3}, 30, 40]]', '$[2][*]');
 json_extract('[10, 20, [{"a":3}, 30, 40]]', '$[2][*]')
 [{"a":3}, 30, 40]
+select json_extract('1', '$');
+json_extract('1', '$')
+1
 select json_insert('{"a":1, "b":{"c":1}, "d":[1, 2]}', '$.b.k1', 'word');
 json_insert('{"a":1, "b":{"c":1}, "d":[1, 2]}', '$.b.k1', 'word')
 {"a":1, "b":{"c":1, "k1":"word"}, "d":[1, 2]}

mysql-test/t/func_json.test

@@ -54,6 +54,7 @@ select json_extract('{"key0":true, "key1":"qwe"}', "$.key1");
 select json_extract(json_object('foo', 'foobar'),'$');
 select json_extract('[10, 20, [30, 40]]', '$[2][*]');
 select json_extract('[10, 20, [{"a":3}, 30, 40]]', '$[2][*]');
+select json_extract('1', '$');
 
 select json_insert('{"a":1, "b":{"c":1}, "d":[1, 2]}', '$.b.k1', 'word');
 select json_insert('{"a":1, "b":{"c":1}, "d":[1, 2]}', '$.d[3]', 3);

sql/item_jsonfunc.cc

@@ -476,9 +476,6 @@ String *Item_func_json_extract::val_str(String *str)
         v_len= je.s.c_str - value;
       }
 
-      if (json_scan_next(&je) && je.s.error)
-        goto error;
-
       if (!multiple_values_found)
       {
         if (first_value == NULL)
@@ -489,7 +486,6 @@ String *Item_func_json_extract::val_str(String *str)
              */
           first_value= (const char *) value;
           first_len= v_len;
-          continue;
         }
         else
         {
@@ -500,15 +496,20 @@ String *Item_func_json_extract::val_str(String *str)
         }
 
       }
-      if (str->append(", ", 2) ||
-          str->append((const char *) value, v_len))
+      if (multiple_values_found &&
+          (str->append(", ", 2) ||
+           str->append((const char *) value, v_len)))
         goto error; /* Out of memory. */
-    }
 
-    if (je.s.error)
-      goto error;
+      if (json_scan_next(&je))
+        break;
+
+    }
   }
 
+  if (je.s.error)
+    goto error;
+
   if (first_value == NULL)
   {
     /* Nothing was found. */