ppapi: fix crash/infinite loop in VideoEncoder

Depending on the libc used with your NaCl plugins, if the VideoEncoder
proxy receives a EncodeReply message after it has been closed, the
plugin will crash (newlib) or enter an infinite loop (glibc) because
it tries to remove the end() iterator from a std::map.

BUG=455409
TEST=run ppapi/examples/video_encode multiple times and verify the test keeps recording new videos

Review URL: https://codereview.chromium.org/1138483002

Cr-Commit-Position: refs/heads/master@{#328997}

ppapi/proxy/video_encoder_resource.cc

@@ -361,7 +361,12 @@ void VideoEncoderResource::OnPluginMsgEncodeReply(
     PP_Resource video_frame,
     const ResourceMessageReplyParams& params,
     uint32_t frame_id) {
-  DCHECK_NE(encode_callbacks_.size(), 0U);
+  // We need to ensure there are still callbacks to be called before
+  // processing this message. We might receive a EncodeReply message
+  // after having sent a Close message to the renderer. In this case,
+  // we don't have any callback left to call.
+  if (encode_callbacks_.empty())
+    return;
   encoder_last_error_ = params.result();
 
   EncodeMap::iterator it = encode_callbacks_.find(video_frame);

ppapi/proxy/video_encoder_resource_unittest.cc

@@ -1136,6 +1136,14 @@ TEST_F(VideoEncoderResourceTest, Close) {
     ASSERT_EQ(PP_ERROR_ABORTED, encode_cb2.result());
     ASSERT_TRUE(get_bitstream_buffer_cb.called());
     ASSERT_EQ(PP_ERROR_ABORTED, get_bitstream_buffer_cb.result());
+
+    // Verify that a remaining encode response from the renderer is
+    // discarded.
+    ResourceMessageCallParams params;
+    uint32_t frame_id;
+    bool force_frame;
+    ASSERT_TRUE(CheckEncodeMsg(&params, &frame_id, &force_frame));
+    SendEncodeReply(params, frame_id);
   }
 }