Remove some calls to malloc_failure

Remove some calls to malloc_failure

I noticed a couple of spots that call malloc_failure, but that don't
need to.

* In xml-support.c, "concat" uses xmalloc, so cannot return NULL.

* In utils.c, "buildargv" also uses xmalloc, so can only return NULL
  if the argument is empty.

Tested by the buildbot.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tromey@adacore.com>

	* xml-support.c (xml_fetch_content_from_file): Don't call
	malloc_failure.
	* utils.h (class gdb_argv): Remove malloc_failure comment.
	* utils.c (gdb_argv::reset): Don't call malloc_failure.

Change-Id: I59483620deb6609ccf2f024d94a29113bb62d1a9