MINOR: stick-tables: make stktable_release() do nothing on NULL

MINOR: stick-tables: make stktable_release() do nothing on NULL

stktable_release() has been involved in two recent crashes by being
used without enough care. Just like any free() function this one is
often called on an exit path with a possibly unsafe argument. Given
that there is another case (smp_fetch_sc_trackers()) which theorically
could call it with an unchecked NULL, though it cannot happen since
the function doesn't support being called with src_* hence cannot make
use of tmpstkctr, let's rather move the check into the function itself
to make it safer for the long term.

This patch could be backported to 1.8 as a strengthening measure.