Fix bug with MDC in pgp-info.c

I had originally thought that it's OK not to care about the MDC if we
skip over the data, but that appears to not be exactly true.  There's
some corner case where ignoring the MDC in a "stream" context would
cause the compressed data packet to be only partially read, resulting in
weird errors about packet headers.  Fix by fully handling MDC, just like
pgp-encrypt.c does.