glob: pacify fuzzer for mempcpy

Problem reported by Tim Rühsen [1].  Sync with gnulib 0e14f025d2.

[1] https://lists.gnu.org/archive/html/bug-gnulib/2017-10/msg00054.html



Checked on x86_64-linux-gnu.

    * lib/glob.c (glob): Do not pass NULL to mempcpy.

Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>