BUG/MINOR: tools: fix possible null-deref in env_expand() on out-of-memory

BUG/MINOR: tools: fix possible null-deref in env_expand() on out-of-memory

In GH issue #2586 @Bbulatov reported a theoretical null-deref in
env_expand() in case there's no memory anymore to expand an environment
variable. The function should return NULL in this case so that the only
caller (str2sa_range) sees it. In practice it may only happen during
boot thus is harmless but better fix it since it's easy. This can be
backported to all versions where this applies.