SUNRPC: Fix a pipe_version reference leak

In gss_alloc_msg(), if the call to gss_encode_v1_msg() fails, we
want to release the reference to the pipe_version that was obtained
earlier in the function.

Fixes: 9d3a2260 (SUNRPC: Fix buffer overflow checking in...)
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>