Bug 1752326 - Prevent deadlock when stopping V4L2 capture. r=ng

This is effectively a cherry-pick of the upstream commit https://webrtc.googlesource.com/src/+/974f6c64388614c332671c98dec8fffc86f35acf . The effected code was a bit different upstream, so there are minor differences.

Differential Revision: https://phabricator.services.mozilla.com/D138175

third_party/libwebrtc/modules/video_capture/linux/video_capture_linux.cc

@@ -386,18 +386,6 @@ bool VideoCaptureModuleV4L2::CaptureProcess() {
   rSet.revents = 0;
 
   retVal = poll(&rSet, 1, 1000);
-  if (retVal < 0 && errno != EINTR)  // continue if interrupted
-  {
-    // poll failed
-    return false;
-  } else if (retVal == 0) {
-    // poll timed out
-    return true;
-  } else if (!(rSet.revents & POLLIN)) {
-    // not event on camera handle
-    return true;
-  }
-
   {
     MutexLock lock(&capture_lock_);
 
@@ -405,6 +393,18 @@ bool VideoCaptureModuleV4L2::CaptureProcess() {
       return false;
     }
 
+    if (retVal < 0 && errno != EINTR)  // continue if interrupted
+    {
+      // poll failed
+      return false;
+    } else if (retVal == 0) {
+      // poll timed out
+      return true;
+    } else if (!(rSet.revents & POLLIN)) {
+      // not event on camera handle
+      return true;
+    }
+
     if (_captureStarted) {
       struct v4l2_buffer buf;
       memset(&buf, 0, sizeof(struct v4l2_buffer));