shared.c: Only setenv() if value is non-null

shared.c: Only setenv() if value is non-null

Some setenv() implementations (e.g. the one in OpenBSD's stdlib)
segfault if we pass a NULL value. Only set environment variables if the
corresponding settings are defined to avoid this.

Note that this is a minor behaviour change as environment variables were
supposed to be set to an empty string if a setting was undefined. Given
that this feature isn't part of any official release yet, there's no
need to worry about backwards compatibility, really. Change the
documentation accordingly.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>