[krbdev.mit.edu #6487] gss_unwrap_iov fails in stream mode

[Tom Yu via RT]

Using IOV_SHIM_EXERCISE results in segfaults in kg_unseal_stream_iov during gss_unwrap 
operations.  Refining the test by only using the IOV_SHIM for gss_wrap causes no problem.  One 
of the segfaults is from a null pointer dereference of ctx->enc.