Fix segfault if target_fileio_read_alloc fails

Check for target_fileio_read_alloc failure in linux_fill_prpsinfo
before dereferencing buffer. This fixes a segfault in the 'gcore'
command when attached to certain remote targets.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32441
Approved-By: Andrew Burgess <aburgess@redhat.com>