fix notmuch_message_file_get_header to always return the first instance
of the header you are looking for
Signed-off-by: Dirk Hohndel <hohndel@infradead.org>
Calling continue here worked only because we set a flag before the
continue, and, check the flag at the beginning of the loop, and *then*
break. It's much more clear to just break in the first place.
