rpcrt4: Check for null endpoint in RpcServerUseProtseqEpExW

[Luke Kenneth Casson Leighton]

see implementation in dual BSD-licensed _and_ LGPL-licensed freedce
reference implementation, on which MSRPC was originally blatantly
copied (thanks to the BSD license on the 1.0 reference
implementation).

e.g. online annotated copy here:
http://www.sfr-fresh.com/linux/misc/freedce-1.1.0.7.tar.gz:a/freedce-1.1.0.7/ncklib/com/comnet.c

line 2470.

anyone who is doing wine dce/rpc development _really_ needs to be
looking at the reference implementation, side-by-side, line-for-line,
in virtually every aspect except for exception handling, threading,
binary-interoperability with the typelibraries and a few other areas.

l.