Fix syntax bugs involving escaped newlines in comments

Fix syntax bugs involving escaped newlines in comments

This fixes bug#63535

* src/syntax.c (forw_comment): take the initial syntax from the
argument PREV_SYNTAX rather than checking the buffer position
for a backslash.  Update the state at each character scanned.
(scan_sexps_forward): When the end position is escaped after a
call to forw_comment, return this status to the caller.