MDEV-12633 Error from valgrind related to dd_frm_type

"Conditional jump or move depends on uninitialised value in
my_scan_weight_utf8_general_ci, main.mysql_client_test fails in biuldbot
with valgrind"

- Fixed by ensuring that engine_name is set to empty string even in case
  errors in the .frm file
- Added some error checking to ha_table_exists()

sql/datadict.cc

@@ -45,6 +45,8 @@ static int read_string(File file, uchar**to, size_t length)
 
   engine_name is a LEX_STRING, where engine_name->str must point to
   a buffer of at least NAME_CHAR_LEN+1 bytes.
+  If engine_name is 0, then the function will only test if the file is a
+  view or not
 
   @retval  FRMTYPE_ERROR        error
   @retval  FRMTYPE_TABLE        table
@@ -72,12 +74,23 @@ frm_type_enum dd_frm_type(THD *thd, char *path, LEX_STRING *engine_name)
     goto err;
   }
 
+  /*
+    We return FRMTYPE_TABLE if we can read the .frm file. This allows us
+    to drop a bad .frm file with DROP TABLE
+  */
   type= FRMTYPE_TABLE;
 
-  if (!is_binary_frm_header(header) || !engine_name)
+  /* engine_name is 0 if we only want to know if table is view or not */
+  if (!engine_name)
     goto err;
 
+  /* Initialize engine name in case we are not able to find it out */
   engine_name->length= 0;
+  engine_name->str[0]= 0;
+
+  if (!is_binary_frm_header(header))
+    goto err;
+
   dbt= header[3];
 
   /* cannot use ha_resolve_by_legacy_type without a THD */

sql/handler.cc

@@ -5063,10 +5063,15 @@ bool ha_table_exists(THD *thd, const char *db, const char *table_name,
     {
       char engine_buf[NAME_CHAR_LEN + 1];
       LEX_STRING engine= { engine_buf, 0 };
+      frm_type_enum type;
 
-      if (dd_frm_type(thd, path, &engine) != FRMTYPE_VIEW)
+      if ((type= dd_frm_type(thd, path, &engine)) == FRMTYPE_ERROR)
+        DBUG_RETURN(0);
+
+      if (type != FRMTYPE_VIEW)
       {
-        plugin_ref p=  plugin_lock_by_name(thd, &engine,  MYSQL_STORAGE_ENGINE_PLUGIN);
+        plugin_ref p= plugin_lock_by_name(thd, &engine,
+                                          MYSQL_STORAGE_ENGINE_PLUGIN);
         *hton= p ? plugin_hton(p) : NULL;
         if (*hton)
           // verify that the table really exists