vibra-pattern: End pending vibra when a new one is triggered

This wasn't much of an issue for the DBus API but when using
it as event feedback it matters as we might start the new
feedback just before the old one ended due to timers being
slighlty late.

src/fbd-feedback-vibra-pattern.c

@@ -192,7 +192,9 @@ fbd_feedback_vibra_pattern_start_vibra (FbdFeedbackVibra *vibra)
   g_return_if_fail (self->magnitudes);
   g_return_if_fail (self->durations);
   g_return_if_fail (self->durations->len == self->magnitudes->len);
-  g_return_if_fail (self->pos == 0);
+
+  if (self->pos)
+    fbd_feedback_vibra_pattern_end_vibra (FBD_FEEDBACK_VIBRA (self));
 
   g_debug ("Pattern Vibra: %u elements", self->durations->len);