Add minor fixes to video_capture_ios.mm in order to make it more robust.

R=tkchin@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46429005

Patch from Iurii Shevchuk <youwrk@gmail.com>.

Cr-Commit-Position: refs/heads/master@{#8763}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8763 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/modules/video_capture/ios/video_capture_ios.mm b/webrtc/modules/video_capture/ios/video_capture_ios.mm
index 2010f03..e9c7763 100644
--- a/webrtc/modules/video_capture/ios/video_capture_ios.mm
+++ b/webrtc/modules/video_capture/ios/video_capture_ios.mm
@@ -31,11 +31,13 @@
   capability_.width = kDefaultWidth;
   capability_.height = kDefaultHeight;
   capability_.maxFPS = kDefaultFrameRate;
+  capture_device_ = nil;
 }
 
 VideoCaptureIos::~VideoCaptureIos() {
   if (is_capturing_) {
     [capture_device_ stopCapture];
+    capture_device_ = nil;
   }
 }