Fix flowbits sigmatch structure added to the match and post-match list, causing corruption of the prev ptr. This lead to an endless loop condition in the thresholding code. Thanks to Chris Wakelin for reporting the issue.

src/detect-flowbits.c

@@ -258,6 +258,7 @@ int DetectFlowbitSetup (DetectEngineCtx *de_ctx, Signature *s, char *rawstr)
         case DETECT_FLOWBITS_CMD_ISSET:
             /* checks, so packet list */
             SigMatchAppendPacket(s, sm);
+            break;
 
         case DETECT_FLOWBITS_CMD_SET:
         case DETECT_FLOWBITS_CMD_UNSET:

src/detect-parse.c

@@ -1439,6 +1439,19 @@ static int SigValidate(Signature *s) {
         }
     }
 
+#ifdef DEBUG
+    int i;
+    for (i = 0; i < DETECT_SM_LIST_MAX; i++) {
+        if (s->sm_lists[i] != NULL) {
+            SigMatch *sm;
+            for (sm = s->sm_lists[i]; sm != NULL; sm = sm->next) {
+                BUG_ON(sm == sm->prev);
+                BUG_ON(sm == sm->next);
+            }
+        }
+    }
+#endif
+
     SCReturnInt(1);
 }