detect/port: add BUG_ON on impossible condition

to assist Coverity in assessing the state of x correctly.

src/detect-engine-port.c

@@ -807,6 +807,7 @@ static int DetectPortParseDo(const DetectEngineCtx *de_ctx,
         } else if (!o_set && s[u] == '!') {
             SCLogDebug("negation encountered");
             n_set = 1;
+            BUG_ON(x == 0); /* always false; added to guide static code analyzers like Coverity */
             x--;
         } else if (s[u] == '[') {
             if (!o_set) {