elan: Do not leak converted frames

The elan driver converts frames into a different format. These frames
are only needed to assemable the image and should be free'ed afterwards.

Fixes: #213

libfprint/drivers/elan.c

@@ -321,6 +321,8 @@ elan_submit_image (FpImageDevice *dev)
   fpi_do_movement_estimation (&assembling_ctx, frames);
   img = fpi_assemble_frames (&assembling_ctx, frames);
 
+  g_slist_free_full (frames, g_free);
+
   fpi_image_device_image_captured (dev, img);
 }