terminal: fix uninitialized variable in strerror() log message

We currently print weird error-messages if xkbcommon fails (which cannot
fail so far, but might in the future). Fix the uninitialized variable
warnings by setting 'r' correctly.
Thanks to Philippe De Swert for catching this (via coverity).

src/libsystemd-terminal/idev-keyboard.c

@@ -770,8 +770,10 @@ static int keyboard_feed_evdev(idev_keyboard *k, idev_data *data) {
                 /* TODO: update LEDs */
         }
 
-        if (num < 0)
+        if (num < 0) {
+                r = num;
                 goto error;
+        }
 
         r = keyboard_fill(k, &k->evdata, data->resync, ev->code, ev->value, num, keysyms);
         if (r < 0)