Commit 1335af4b51 restructured briefing number parsing, but
unintentionally changed the parser not to drop the newline after a
number. The briefing parsing code is very sensitive to minor changes,
so this broke parsing some briefing screens, including Vertigo briefing
screens after the first. After the change, clicking through the Vertigo
briefing aborts on failure to load a PCX file.
Fix this by consuming the newline before returning. This matches the
previous semantics.
Reported-by: Havner <https://github.com/dxx-rebirth/dxx-rebirth/issues/343>
Fixes: 1335af4b51 ("Simplify get_message_num")
7c0cdd6c51