preproc: parse_size -- Check for string provided
preproc: parse_size -- Check for string provided

In case if the string is nil we will have sigsegv.

https://bugzilla.nasm.us/show_bug.cgi?id=3392507

Signed-off-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@gmail.com>

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