afl - Don't fail if app-layer proto enabled value is NULL.

Found by using AFL on suricata.yaml.

src/app-layer-detect-proto.c

@@ -1630,16 +1630,19 @@ int AppLayerProtoDetectConfProtoDetectionEnabled(const char *ipproto,
         }
     }
 
-    if (strcasecmp(node->val, "yes") == 0) {
+    if (node->val) {
+        if (ConfValIsTrue(node->val)) {
             goto enabled;
-    } else if (strcasecmp(node->val, "no") == 0) {
+        } else if (ConfValIsFalse(node->val)) {
             goto disabled;
         } else if (strcasecmp(node->val, "detection-only") == 0) {
             goto enabled;
-    } else {
+        }
+    }
+
+    /* Invalid or null value. */
     SCLogError(SC_ERR_FATAL, "Invalid value found for %s.", param);
     exit(EXIT_FAILURE);
-    }
 
  disabled:
     enabled = 0;