Add some checks if the original/dup'd fd is valid and ensure that we
don't leak it on error. The former is implicitly handled within the
pipe_loader, although let's make things explicit and check beforehand.

Spotted by Coverity (CID 1339865)

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>