stream-tcp-reassemble: fix reassembly direction for FIN packets

Suricata invokes the stream reassembly logic only for the current packet
direction if the packet contains a FIN flag. However, this does not
handle the case in which the packet ACKs data from the opposing direction.
This patch forces the invocation of the stream reassembly logic
on both direction when Suricata sees a FIN packet.

src/stream-tcp-reassemble.c

@@ -1853,7 +1853,11 @@ int StreamTcpReassembleHandleSegment(ThreadVars *tv, TcpReassemblyThreadCtx *ra_
     } else if (p->tcph->th_flags & TH_RST) { // accepted rst
         dir = UPDATE_DIR_PACKET;
     } else if ((p->tcph->th_flags & TH_FIN) && ssn->state > TCP_TIME_WAIT) {
-        dir = UPDATE_DIR_PACKET;
+        if (p->tcph->th_flags & TH_ACK) {
+            dir = UPDATE_DIR_BOTH;
+        } else {
+            dir = UPDATE_DIR_PACKET;
+        }
     } else if (ssn->state == TCP_CLOSED) {
         dir = UPDATE_DIR_BOTH;
     }