fixes bug 256316 "non-ascii char in URL lead to heap overrun" r=dveditz sr=bryner

netwerk/dns/src/nsIDNService.cpp

@@ -241,6 +241,9 @@ NS_IMETHODIMP nsIDNService::IsACE(const nsACString & input, PRBool *_retval)
 
 NS_IMETHODIMP nsIDNService::Normalize(const nsACString & input, nsACString & output)
 {
+  // protect against bogus input
+  NS_ENSURE_ARG(IsUTF8(input));
+
   nsAutoString outUTF16;
   nsresult rv = stringPrep(NS_ConvertUTF8toUTF16(input), outUTF16);
   if (NS_SUCCEEDED(rv))