In TestExternalUnmount, the Mount function is called which returns an
error which can be nil. The error type is then used in a comparison
where Error() is called on it. If the error is nil, this results in a
panic.

Added a if err != nil {} guard to make sure that Error() is not called
if the value is nil
 On branch go-test-fix
 Changes to be committed:
	modified:   fuse/node/mount_test.go
License: MIT
Signed-off-by: Chris Buesser <christopher.buesser@gmail.com>