lib/mbsalign: abort() when non-expected case is encountered

Fixes the following compiler warning (9 occurrences), and makes program
to crash if mbsaligh() is called with unexpected align value (which
would be programming error).

../lib/mbsalign.c:260:7: warning: switch missing default case [-Wswitch-default]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

lib/mbsalign.c

@@ -271,6 +271,8 @@ mbsalign_unibyte:
           start_spaces = n_spaces;
           end_spaces = 0;
           break;
+	default:
+	  abort();
         }
 
       dest = mbs_align_pad (dest, dest_end, start_spaces);