Bug 57347 --> add a null ptr check to prevent a crash.

patch contributed by waterson
r=brendan
sr=mscott

uriloader/exthandler/unix/nsOSHelperAppService.cpp

@@ -73,12 +73,14 @@ NS_IMETHODIMP nsOSHelperAppService::DoContent(const char *aMimeContentType, nsIU
   if (aURI)
   {
     nsCOMPtr<nsIURL> url = do_QueryInterface(aURI);
+    if (url) {
       nsXPIDLCString extenion;
       url->GetFileExtension(getter_Copies(extenion));
     
       fileExtension = ".";  
       fileExtension.Append(extenion);
     }
+  }
 
   if (mimeInfo)
   {