MDEV-11601 Out-of-bounds string access in create_schema_table()

in Item_partition_func_safe_string(THD *thd, const char *name_arg,
uint length, CHARSET_INFO *cs= NULL), the 'name_arg' is the value
of the string constant and 'length' is the length of this constant,
so length == strlen(name_arg).

sql/item.h

@@ -3206,7 +3206,7 @@ class Item_blob :public Item_partition_func_safe_string
 {
 public:
   Item_blob(THD *thd, const char *name_arg, uint length):
-    Item_partition_func_safe_string(thd, name_arg, length, &my_charset_bin)
+    Item_partition_func_safe_string(thd, name_arg, strlen(name_arg), &my_charset_bin)
   { max_length= length; }
   enum Type type() const { return TYPE_HOLDER; }
   enum_field_types field_type() const { return MYSQL_TYPE_BLOB; }