Check for null context

https://bugs.webkit.org/show_bug.cgi?id=69149
describes a situation where running webkit tests
sporadically crashes in the Debug build.

The context returned by CGContextForData is NULL,
so an early return prevents the BitmapPlatformDevice
constructor from firing an assert in this case.

The callers already anticipate a NULL result, and
will propogate the error correctly.

BUG=WK69149
TEST=run webkit tests in a Debug build repeatedly

Review URL: http://codereview.chromium.org/8118014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@104374 0039d316-1c4b-4281-b951-d872f2087c98

skia/ext/bitmap_platform_device_mac.cc

@@ -143,9 +143,11 @@ BitmapPlatformDevice* BitmapPlatformDevice::Create(CGContextRef context,
   }
 #endif
 
-  if (!context)
+  if (!context) {
     context = CGContextForData(data, width, height);
-  else
+    if (!context)
+      return NULL;
+  } else
     CGContextRetain(context);
 
   BitmapPlatformDevice* rv = new BitmapPlatformDevice(