crash at registration

[Philippe Torrelli]

Hello, 

Testing minisip register against a openser proxy here  ( this is with v3157)

At register, if there are no credentials in the configuration file, but the
registrar asks for an authentication,  minisip simply crashes ... 

I think it's because getCredential returns NULL in 
SipDialog::updateAuthentication ... 
	challenge->setCredential(
getDialogConfig()->sipIdentity->getCredential()  );  
 

Philippe Torrelli