xalloc: pacify -Wanalyzer-possible-null-argument Problem reported for GCC 10.1.0 by Bruno Haible in: https://lists.gnu.org/r/bug-gnulib/2020-05/msg00118.html * lib/xmalloc.c (HAVE_GNU_MALLOC, HAVE_GNU_REALLOC): New constants. (xmalloc): Suppress unnecessary check if HAVE_GNU_MALLOC. (xrealloc): Suppress unnecssary check if HAVE_GNU_REALLOC.