assemble: don't pass a NULL filename to dfmt->linenum
assemble: don't pass a NULL filename to dfmt->linenum

Don't pass a NULL filename to dfmt->linenum even if -Lb is in use; it
confuses the heck out of some debug backends.

Signed-off-by: H. Peter Anvin (Intel) <8a453bad9912ffe59bc0f0b8abe03df9be19379e@zytor.com>

techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm,techkey/nasm