gdb: address review comments of previous series

gdb: address review comments of previous series

I forgot to include fixes for review comments I got before pushing the
previous commits (or I pushed the wrong commits).  This one fixes it.

 - Return {} instead of false in get_discrete_low_bound and
   get_discrete_high_bound.
 - Compute high bound after confirming low bound is valid in
   get_discrete_bounds.

gdb/ChangeLog:

	* gdbtypes.c (get_discrete_low_bound, get_discrete_high_bound):
	Return {} instead of false.
	(get_discrete_bounds): Compute high bound only if low bound is
	valid.

Change-Id: I5f9a66b3672adfac9441068c899ab113ab2c331a