lib-dcrypt: Avoid infinite loop if istream header is too large.

lib-dcrypt: Avoid infinite loop if istream header is too large.

We'll return an error now instead. We can't just return -2 here, because nothing
was actually being returned to the caller. Attempting to do that would just trigger
an assert:

Panic: file istream.c: line 182 (i_stream_read): assertion failed: (_stream->skip != _stream->pos)