cgit.c: Do not restore unset environment variables

getenv() returns a NULL pointer if the specified variable name cannot be
found in the environment. However, some setenv() implementations crash
if a NULL pointer is passed as second argument. Only restore variables
that are not NULL.

See commit d96d2c98ebc4c2d3765f5b35c4142e0e828a421b for a related patch.

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