detect: address intermittent UT crash on OpenBSD

Add missing flow memset in test SCSigOrderingTest12 to avoid the crash.

Test SCSigOrderingTest12                                          : Info: unittest-helper: Sid 1 matched 1 times, as expected [UTHCheckPacketMatchResults:util-unittest-helper.c:638]
Info: unittest-helper: Sid 2 matched 1 times, as expected [UTHCheckPacketMatchResults:util-unittest-helper.c:638]
bash: line 144: 88567 Segmentation fault      (core dumped) ./src/suricata -u -l /tmp/

Bug: #8008.
(cherry picked from commit 232f0572e0)

src/detect-engine-sigorder.c

@@ -1997,7 +1997,7 @@ static int SCSigOrderingTest12(void)
     Packet *p = NULL;
     uint8_t buf[] = "test message";
     Flow f;
-
+    memset(&f, 0, sizeof(f));
     FLOW_INITIALIZE(&f);
     f.flags |= FLOW_IPV4;
     f.alproto = ALPROTO_UNKNOWN;