[gdb/tui] Fix buglet in tui_update_variables

[gdb/tui] Fix buglet in tui_update_variables

I noticed a buglet in tui_update_variables:
...
   entry = translate (tui_border_kind, tui_border_kind_translate_lrcorner);
   if (tui_border_lrcorner != (chtype) entry->value)
    {
      tui_border_lrcorner = (entry->value < 0) ? ACS_LRCORNER : entry->value;
...

When assigning the new value to tui_border_lrcorner, an entry->value of -1 is
taken into account, but not when comparing to the current value of
tui_border_lrcorner.

Fix this by introducing:
...
  int val = (entry->value < 0) ? ACS_LRCORNER : entry->value;
...
and using this in both comparison and assignment.

Tested on x86_64-linux.