Added some checking that LEX_CSTRING is \0 terminated

- When adding LEX_CSTRING to String, we are now checking that
  string is \0 terminated (as normally LEX_CSTRING should be
  usable for printf(). In the cases when one wants to avoid the
  checking one can use String->append(ptr, length) instead of just
  String->append(LEX_CSTRING*)

sql/sql_partition.cc

@@ -7217,7 +7217,7 @@ void append_row_to_str(String &str, const uchar *row, TABLE *table)
   {
     Field *field= *field_ptr;
     str.append(" ");
-    str.append(field->field_name);
+    str.append(&field->field_name);
     str.append(":");
     field_unpack(&str, field, rec, 0, false);
   }