GIF: Fix the previous frame not always being decoded in _seek.

_seek checked whether self.im is None, but if we've decoded a frame
and then seeked back to 0, self.im will be set to the previously
decoded frame.  Instead, check if self.tile has data, which means
_seek set up a tile to decode and it hasn't been decoded yet.

Tests/test_file_gif.py

@@ -293,6 +293,26 @@ class TestFileGif(PillowTestCase):
                 self.assertEqual(color, expected_colors[frame][x],
                     'frame %i, x %i' % (frame, x))
 
+    def test_rewind(self):
+        img = Image.open("Tests/images/rewind.gif")
+
+        # Seek forwards to frame 2.  This will decode frames 0 and 1.
+        img.seek(2)
+
+        # Seek back to frame 1.  This will rewind to frame 0, then seek
+        # to frame 1.
+        img.seek(1)
+
+        # Read data.  This should decode frame 0 and then frame 1.
+        img.getpixel((0,1))
+
+        expected_colors = [(2, 3), (3, 1)]
+        for x in range(2):
+            for y in range(2):
+                color = img.getpixel((x,y))
+                self.assertEqual(color, expected_colors[x][y],
+                    '%ix%i' % (x, y))
+
     def test_dispose_previous(self):
         img = Image.open("Tests/images/dispose_prev.gif")
         try:

src/PIL/GifImagePlugin.py

@@ -144,7 +144,7 @@ class GifImageFile(ImageFile.ImageFile):
             self.disposal_method = 0
         else:
             # ensure that the previous frame was loaded
-            if not self.im:
+            if self.tile:
                 self.load()
 
         if frame != self.__frame + 1: