Avoid crash in compile_to_object

Avoid crash in compile_to_object

PR 23672 points out a crash in compile_to_object.  This crash came in
during a C++-ization.  This patch avoids the crash.

The PR also points out another weird behavior in this code, but that
one requires some setup that I don't have here, and it seems to date
back to the introduction of the compile feature.  So, it isn't
addressed here.  I will leave the PR open so this bug isn't forgotten.

gdb/ChangeLog
2021-01-09  Tom Tromey  <tom@tromey.com>

	PR compile/23672
	* compile/compile.c (compile_to_object): Avoid crash when
	osabi_triplet_regexp returns NULL.