loader: we can only env_discard() existing variable

While dropping nvpair from nvstore, we also remove the corresponding
environment variable. By doing so, we should be careful not to try
to unset non-existing variable.

Reviewed by: imp
MFC after: 2 week
Differential revision: https://reviews.freebsd.org/D44083

(cherry picked from commit 1b3f4ccb7dd8600d010fc6a09b09ee7d74872809)