rpcrt4: Fix context handle test on Vista upwards.

[Rob Shearman]

Don't run tests that results in an ERROR_INVALID_HANDLE exception being raised .

Check for the context handle function pointers being non-NULL to
prevent exceptions being raised on platforms without these functions.
---
 dlls/rpcrt4/tests/server.c |   10 ++++++++++
 1 files changed, 10 insertions(+), 0 deletions(-)

I wasn't able to determine the cause of the failure of the test that I
disabled on Vista without running it under a debugger and possibly
stepping through native code. Since we have other tests of this code
in Wine in the form of the running object table and services RPC
servers it isn't important for this test to fixed instead of disabled.
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