Panic in WR when updating media image

[jdm]

I hit this when closing the reddit homepage after scrolling around.

Attempt to update non-existent image (thread WRRenderBackend#0, at /Users/servo/.cargo/git/checkouts/webrender-c3596abe1cf4f320/2ff8da0/webrender/src/resource_cache.rs:863)
Stack trace
stack backtrace:
   0:        0x10fe611de - __ZN9backtrace9backtrace5trace17h4c592a1c902b528fE
   1:        0x10fe6011c - __ZN72_$LT$backtrace..capture..Backtrace$u20$as$u20$core..default..Default$GT$7default17hbd9a991ebe7fe79cE
   2:        0x10dcb1264 - __ZN5servo21install_crash_handler7handler17h2b84bb8b3c679b14E
   3:     0x7fffd81e1b39 - __sigtramp
   4:        0x1106fb890 - __Z14mozalloc_abortPKc
   5:        0x1106fb8ac - _abort
   6:        0x1108d7b88 - __ZN3std3sys4unix14abort_internal17hb4a4d7d00017fd0aE
   7:        0x1108cda4a - __ZN3std10sys_common4util5abort17h4c7337aecb60e835E
   8:        0x1108cf3fd - _rust_panic
   9:        0x1108cf2c8 - __ZN3std9panicking20rust_panic_with_hook17hefe388a303cb8993E
  10:        0x1108cecbc - __ZN3std9panicking18continue_panic_fmt17hfb4dee9aa0dc869fE
  11:        0x1108ceba8 - _rust_begin_unwind
  12:        0x1108ec801 - __ZN4core9panicking9panic_fmt17h921c11dbb9b2cd6cE
  13:        0x10feb73b0 - __ZN4core6result13unwrap_failed17hee099147872640dcE
  14:        0x10fe9f1f4 - __ZN13webrender_api3api9RenderApi16update_resources17hdae58bb28756e8f5E
  15:        0x10e8536ed - __ZN110_$LT$script..dom..htmlmediaelement..MediaFrameRenderer$u20$as$u20$servo_media_player..frame..FrameRenderer$GT$6render17h5522d938ace4f4c1E
  16:        0x10eeb9ac0 - __ZN21servo_media_gstreamer6player17FrameRendererList6render17hbd6fc8df8308f49dE
  17:        0x10eee5951 - __ZN21servo_media_gstreamer6player15GStreamerPlayer5setup28_$u7b$$u7b$closure$u7d$$u7d$17h06dc2be9d139eabfE
  18:        0x10eee890f - __ZN13gstreamer_app8app_sink21trampoline_new_sample17h2b2f87fd84b17160E
  19:        0x1138b9a09 - _gst_app_sink_render_common
  20:        0x11358dd44 - _gst_base_sink_chain_unlocked
  21:        0x11359202b - _gst_base_sink_chain_main
  22:        0x11361f983 - _gst_pad_chain_data_unchecked
  23:        0x1136203e0 - _gst_pad_push_data
  24:        0x11362020a - _gst_pad_push
  25:        0x11360b871 - _gst_proxy_pad_chain_default
  26:        0x11361f983 - _gst_pad_chain_data_unchecked
  27:        0x1136203e0 - _gst_pad_push_data
  28:        0x11362020a - _gst_pad_push
  29:        0x11359fe5b - _gst_base_transform_chain
  30:        0x11361f983 - _gst_pad_chain_data_unchecked
  31:        0x1136203e0 - _gst_pad_push_data
  32:        0x11362020a - _gst_pad_push
  33:        0x11359fe5b - _gst_base_transform_chain
  34:        0x11361f983 - _gst_pad_chain_data_unchecked
  35:        0x1136203e0 - _gst_pad_push_data
  36:        0x11362020a - _gst_pad_push
  37:        0x11359fe5b - _gst_base_transform_chain
  38:        0x11361f983 - _gst_pad_chain_data_unchecked
  39:        0x1136203e0 - _gst_pad_push_data
  40:        0x11362020a - _gst_pad_push
  41:        0x11359fe5b - _gst_base_transform_chain
  42:        0x11361f983 - _gst_pad_chain_data_unchecked
  43:        0x1136203e0 - _gst_pad_push_data
  44:        0x11362020a - _gst_pad_push
  45:        0x11360b871 - _gst_proxy_pad_chain_default
  46:        0x11361f983 - _gst_pad_chain_data_unchecked
  47:        0x1136203e0 - _gst_pad_push_data
  48:        0x11362020a - _gst_pad_push
  49:        0x1274f99de - _gst_queue_loop
  50:        0x11364db31 - _gst_task_func
  51:        0x113786ec5 - _g_thread_pool_thread_proxy
  52:        0x113786227 - _g_thread_proxy
  53:     0x7fffd81eb93a - __pthread_body
  54:     0x7fffd81eb886 - __pthread_start

[ferjm]

CC @ceyusa