rpcbind: Backport fixes to memory leak security fix

Commit 954509fb added a security fix for CVE-2017-8779, involving
pairing all svc_getargs() calls with svc_freeargs() to avoid a memory
leak.  However it also introduced a couple of issues:

- The call to svc_freeargs() from rpcbproc_callit_com() may result in
  an attempt to free static memory, resulting in undefined behaviour.

- A typo in the svc_freeargs() call from pmapproc_dump() causes NIS
  (aka ypbind) to fail.

Backport upstream fixes for these issues to version 0.2.3.

Signed-off-by: Ed Blake <ed.blake@sondrel.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>