ftp/expectation: fix ippair leak

Ticket: 5204

ippair use_cnt increased by AppLayerExpectationCreate calling
IPPairGetIPPairFromHash was never decreased

(cherry picked from commit 425b9c6777)

src/app-layer-expectation.c

@@ -188,6 +188,7 @@ static ExpectationList *AppLayerExpectationRemove(IPPair *ipp,
 {
     CIRCLEQ_REMOVE(&exp_list->list, exp, entries);
     AppLayerFreeExpectation(exp);
+    IPPairDecrUsecnt(ipp);
     SC_ATOMIC_SUB(expectation_count, 1);
     exp_list->length--;
     if (exp_list->length == 0) {