don't dereference NULL

[Jim Meyering]

In http://git.sv.gnu.org/gitweb/?p=gnulib.git;a=commitdiff;h=92a8f360080d91e9e,
there are three additions like this:

  diff --git a/m4/locale-fr.m4 b/m4/locale-fr.m4
  ...
  +#ifdef __CYGWIN__
  ...
  +  if (strchr (getenv ("LC_ALL"), '.') == NULL) return 1;

That will segfault (dereference NULL) when getenv returns NULL.
Surely that was unintentional...