nasm: avoid null pointer reference on VERY early memory allocation failure
nasm: avoid null pointer reference on VERY early memory allocation failure

If we get a memory allocation failure before preproc is initialized,
we could end up taking a NULL pointer reference while trying to unwind
macros.

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

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