libc/stdlib/getenv.c: always allocate new environment

in particular, if the old environment is NULL.

Among making it less surprising for userspace to observe NULL environ,
the change also prevents NULL deref in __rebuild_environ() when
terminating the empty as NULL environment with the NULL pointer.

Reported by:	 Leo Bicknell <[email protected]>
PR:	298747
Reviewed by:	emaste, markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D59996

lib/libc/stdlib/getenv.c

@@ -268,7 +268,7 @@ __rebuild_environ(int newEnvironSize)
 	int tmpEnvironSize;
 
 	/* Resize environ. */
-	if (newEnvironSize > environSize) {
+	if (newEnvironSize > environSize || intEnviron == NULL) {
 		tmpEnvironSize = newEnvironSize * 2;
 		tmpEnviron = reallocarray(intEnviron, tmpEnvironSize + 1,
 		    sizeof(*intEnviron));