Fix crash in remoteDispatchDomainMemoryStats (CVE-2013-4296)

The 'stats' variable was not initialized to NULL, so if some
early validation of the RPC call fails, it is possible to jump
to the 'cleanup' label and VIR_FREE an uninitialized pointer.
This is a security flaw, since the API can be called from a
readonly connection which can trigger the validation checks.

This was introduced in release v0.9.1 onwards by

  commit 158ba873
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Wed Apr 13 16:21:35 2011 +0100

    Merge all returns paths from dispatcher into single path

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
(cherry picked from commit e7f400a1)

Conflicts:
	daemon/remote.c - context