MDEV-21941: Fix GCC 10 -Wmaybe-uninitialized

commit 105b879d introduced this
warning. The warning looks harmless, but GCC does not understand
that the initialization and the use of the variables are guarded
by the same predicate.

sql/sql_table.cc

@@ -8146,8 +8146,8 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   Field **f_ptr,*field;
   MY_BITMAP *dropped_fields= NULL; // if it's NULL - no dropped fields
   bool drop_period= false;
-  LEX_CSTRING period_start_name;
-  LEX_CSTRING period_end_name;
+  LEX_CSTRING period_start_name= {nullptr, 0};
+  LEX_CSTRING period_end_name= {nullptr, 0};
   if (table->s->period.name)
   {
     period_start_name= table->s->period_start_field()->field_name;