listing: Don't try to open non-specified listing
listing: Don't try to open non-specified listing

By default we setup @listname to '\0' and if has
not been specified in command line it is passed
in this form into list_init() which cause

 | [cyrill@uranus nasm.git] ./nasm -felf64 t.asm
 | nasm: error: unable to open listing file `'

So make a proper test here.

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

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