sunrpc: Avoid use-after-free read access in clntudp_call [BZ #21115]

After commit bc779a1a
(CVE-2016-4429: sunrpc: Do not use alloca in clntudp_call
[BZ #20112]), ancillary data is stored on the heap,
but it is accessed after it has been freed.

The test case must be run under a heap debugger such as valgrind
to observe the invalid access.  A malloc implementation which
immediately calls munmap on free would catch this bug as well.