S_scan_const: Make sure room for NUL in dest

S_scan_const: Make sure room for NUL in dest

At the end of a constant, we add a trailing NUL.  This commit makes sure
there's room for it.  But the code earlier was supposed to have already
made enough space, so its a bug if there isn't enough space.  So on
DEBUGGING builds, we panic, as we've done before.  But otherwise we can
continue on with no actual harm having been done.