detect: remove unneeded NULL check

As spotted by cppcheck

fd is set only once, and DetectFlowSetup bails early if it is NULL

Fixes: c272a646c5 ("detect: SigMatchAppendSMToList can fail")

src/detect-flow.c

@@ -422,7 +422,7 @@ int DetectFlowSetup (DetectEngineCtx *de_ctx, Signature *s, const char *flowstr)
                     de_ctx, s, DETECT_FLOW, (SigMatchCtx *)fd, DETECT_SM_LIST_MATCH) == NULL) {
             goto error;
         }
-    } else if (fd != NULL) {
+    } else {
         DetectFlowFree(de_ctx, fd);
     }