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 <gorcunov@gmail.com>

listing.c

@@ -128,7 +128,7 @@ static void list_emit(void)
 
 static void list_init(const char *fname)
 {
-    if (!fname) {
+    if (!fname || fname[0] == '\0') {
 	listfp = NULL;
 	return;
     }