Bug 1316206 - Fix RefPtr assignments from 0 - r=froydnj

Giving '0' (literal zero) to RefPtr is now ambiguous, as both
RefPtr(decltype(nullptr)) and RefPtr(T*) could be used.
In any case, our coding standards mandate the use of 'nullptr' for pointers.
So I'm changing all zeroes into nullptr's where necessary.

MozReview-Commit-ID: A458A4e9for

netwerk/cache/nsDiskCacheDeviceSQL.cpp

@@ -1413,7 +1413,7 @@ nsOfflineCacheDevice::Shutdown()
   if (NS_FAILED(rv))
     NS_WARNING("Failed to clean up namespaces.");
 
-  mEvictionFunction = 0;
+  mEvictionFunction = nullptr;
 
   mStatement_CacheSize = nullptr;
   mStatement_ApplicationCacheSize = nullptr;