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

[kib]

Summary

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 <[e-mail]>
PR:     298747

[kib]

Always allocate new environment instead of checking it for NULL.