core: Do not crash if CMS plaintext is ignored

* src/engine-gpgsm.c (gpgsm_verify): Fix handling both
plaintext and signed_text as NULL.

Previously if plaintext was NULL and signed_text was NULL
it would set MESSAGE_FD to NULL which resulted in a
crash.

Ignoring the plaintext of an opaque signature might
make sense in some cases and engine-gpg handles it.