gdb/linux-tdep: check return value of linux_find_memory_region_ftype callback

I noticed that linux_find_memory_regions_full did not check the return
value of the linux_find_memory_region_ftype callback.  I think this is a
mistake.  When called from linux_find_memory_regions, the
find_memory_region_ftype callback could return false, in which case we
should stop iterating.

This probably didn't matter in practice, as these callbacks generally
don't return false (only in error cases that never happen).

Change-Id: Iafc5a9aae3d955454420d700a23f18de6f0bc267
Approved-By: Tom Tromey <tom@tromey.com>