Fix _NARG crashing outside of macros

And add a test for it

src/asm/symbol.c

@@ -77,6 +77,10 @@ void sym_ForEach(void (*func)(struct Symbol *, void *), void *arg)
 
 static int32_t Callback_NARG(void)
 {
+	if (!macro_GetCurrentArgs()) {
+		yyerror("_NARG does not make sense outside of a macro");
+		return 0;
+	}
 	return macro_NbArgs();
 }