core: Normalize negative seek positions

This reverts a change between 1.0 and 1.1, so no changelog.

Fixes #1180

mopidy/core/playback.py

@@ -399,7 +399,12 @@ class PlaybackController(object):
         :type time_position: int
         :rtype: :class:`True` if successful, else :class:`False`
         """
-        validation.check_integer(time_position, min=0)
+        validation.check_integer(time_position)
+
+        if time_position < 0:
+            logger.debug(
+                'Client seeked to negative position. Seeking to zero.')
+            time_position = 0
 
         if not self.core.tracklist.tracks:
             return False

tests/core/test_playback.py

@@ -536,6 +536,12 @@ class CorePlaybackTest(unittest.TestCase):
         self.assertFalse(self.playback1.seek.called)
         self.playback2.seek.assert_called_once_with(10000)
 
+    def test_seek_normalizes_negative_positions_to_zero(self):
+        self.core.playback.play(self.tl_tracks[0])
+        self.core.playback.seek(-100)
+
+        self.playback1.seek.assert_called_once_with(0)
+
     def test_seek_fails_for_unplayable_track(self):
         self.set_current_tl_track(self.unplayable_tl_track)
         self.core.playback.state = core.PlaybackState.PLAYING