If the platform regex cannot match null bytes, we might as well
use the glibc version instead.

Cc: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Cc: René Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Tested-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>