gpg: Improve detection of input data read errors.

* g10/build-packet.c (do_plaintext): Better error checking for
iobuf_copy.

The original fix handles only the disk full case but didn't bother
about read errors (i.e. I/O problems on an external drive).

• Fixes-commit: 2fdb950471bd36f046672254ff26ca94797cc9f1
• GnuPG-bug-id: T6528