pki: NULL check pki_signature_from_rsa_blob result

Check for a potential NULL result from pki_signature_from_rsa_blob
in pki_signature_from_blob. Otherwise the following sig->type_c
will result in a segfault.

Introduced in 7f83a1efae6a7da19e18268d6298fc11b4e68c57.

Signed-off-by: Jon Simons <jon@jonsimons.org>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>