acl-plugin: fix failures in some of IPv4 test-debug testcases

Commit 1c7bf5d41737984907e8bad1dc832eb6cb1d6288 added the poisoning
of the newly freed memory in debug builds, exposing a logic
error in mask assignment code - it passed a pointer to
within a pool to a function which might potentially expand the pool.

This resulted in a failure of the test in the debug version.

Fix that by making a local copy of the value before passing
a pointer to it.

Change-Id: I73f3670672c3d86778aad0f944d052d0480cc593
Signed-off-by: Andrew Yourtchenko <ayourtch@gmail.com>