scan.l: fix crash bug in do_atif()

This is really a memory leak because the Unref() call is missing.  But
since this usually returns a "stock" object (`ValManager::b_true` or
`ValManager::b_false`), nothing really leaks.  But eventually, the
reference counter will overflow to `INT_MAX`, leading to a crash in
bad_ref().

src/scan.l

@@ -762,6 +762,8 @@ void do_atif(Expr* expr)
 		if_stack.push_back(current_depth);
 		BEGIN(IGNORE);
 		}
+
+	Unref(val);
 	}
 
 void do_atifdef(const char* id)