fixes bug 168577 "Crash in trunk [@ nsViewSourceChannel::GetURI] viewing

source of a mail message" r=bbaetz sr=bzbarsky

netwerk/protocol/viewsource/src/nsViewSourceChannel.cpp

@@ -178,6 +178,12 @@ nsViewSourceChannel::GetURI(nsIURI* *aURI)
     if (NS_FAILED(rv))
       return rv;
 
+    // protect ourselves against broken channel implementations
+    if (!uri) {
+      NS_ERROR("inner channel returned NS_OK and a null URI");
+      return NS_ERROR_UNEXPECTED;
+    }
+
     nsCAutoString spec;
     uri->GetSpec(spec);