fusefrontend: prepareAtSyscall: handle error when opening ourselves
fusefrontend: prepareAtSyscall: handle error when opening ourselves

Error handling was missing here, so we would later get confusing
EBADF errors due to dirfd being -1.

rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs,rfjakob/gocryptfs