res_pjsip_session: Make reschedule_reinvite check for NULL topologies

When the check for equal topologies was added to reschedule_reinvite()
it was assumed that both the pending and active media states would
actually have non-NULL topologies.  We since discovered this isn't
the case.

We now only test for equal topologies if both media states have
non-NULL topologies.  The logic had to be rearranged a bit to make
sure that we cloned the media states if their topologies were
non-NULL but weren't equal.

ASTERISK-29215

Change-Id: I61313cca7fc571144338aac826091791b87b6e17