libgomp: Handle NULL environ like pointer to NULL pointer [PR111413]

clearenv function just sets environ to NULL (after sometimes freeing it),
rather than setting it to a pointer to NULL, and our code was assuming
it is always non-NULL.

Fixed thusly, the change seems to be large but actually is just
+  if (environ)
     for (env = environ; *env != 0; env++)
plus reindentation.  I've also noticed the block after this for loop
was badly indented (too much) and fixed that too.

No testcase added, as it needs clearenv + dlopen.

2023-09-19  Jakub Jelinek  <jakub@redhat.com>

	PR libgomp/111413
	* env.c (initialize_env): Don't dereference environ if it is NULL.
	Reindent.

(cherry picked from commit 15345980)