Speed up n_frames by skipping past the last frame already seeked

Author: radarhere

src/PIL/TiffImagePlugin.py

@@ -998,6 +998,7 @@ def _open(self):
     def n_frames(self):
         if self._n_frames is None:
             current = self.tell()
+            self._seek(len(self._frame_pos))
             while self._n_frames is None:
                 self._seek(self.tell() + 1)
             self.seek(current)