fix(http): Fix NULL request error under root path

If we send GET request under root URL path, the HTTP
router would not handle it and return segmentation fault. We
need to use function `process_method_not_allowed_request()`
to block this error

connectivity/http/http.c

@@ -295,17 +295,17 @@ static int ta_http_process_request(ta_http_t *const http, char const *const url,
   } else if (ta_http_url_matcher(url, "/mam[/]?") == SC_OK) {
     if (payload != NULL) {
       return process_mam_send_msg_request(http, payload, out);
-    } else {
-      return process_method_not_allowed_request(out);
     }
+    return process_method_not_allowed_request(out);
+
   } else if (ta_http_url_matcher(url, "/transaction/[A-Z9]{81}[/]?") == SC_OK) {
     return process_find_txn_obj_single_request(http, url, out);
   } else if (ta_http_url_matcher(url, "/transaction/object[/]?") == SC_OK) {
     if (payload != NULL) {
       return process_find_txn_obj_request(http, payload, out);
-    } else {
-      return process_method_not_allowed_request(out);
     }
+    return process_method_not_allowed_request(out);
+
   } else if (ta_http_url_matcher(url, "/tips/pair[/]?") == SC_OK) {
     return process_get_tips_pair_request(http, out);
   } else if (ta_http_url_matcher(url, "/tips[/]?") == SC_OK) {
@@ -329,20 +329,23 @@ static int ta_http_process_request(ta_http_t *const http, char const *const url,
   else if (ta_http_url_matcher(url, "/transaction[/]?") == SC_OK) {
     if (payload != NULL) {
       return process_send_transfer_request(http, payload, out);
-    } else {
-      return process_method_not_allowed_request(out);
     }
+    return process_method_not_allowed_request(out);
+
   } else if (ta_http_url_matcher(url, "/tryte[/]?") == SC_OK) {
     if (payload != NULL) {
       return process_send_trytes_request(http, payload, out);
-    } else {
-      return process_method_not_allowed_request(out);
     }
+    return process_method_not_allowed_request(out);
+
   } else if (ta_http_url_matcher(url, "/info[/]?") == SC_OK) {
     return process_get_ta_info_request(http, out);
   } else if (ta_http_url_matcher(url, "/") == SC_OK) {
-    // POST request
-    return process_proxy_api_request(http, payload, out);
+    if (payload != NULL) {
+      return process_proxy_api_request(http, payload, out);
+    }
+    return process_method_not_allowed_request(out);
+
   } else {
     ta_log_error("SC_HTTP_URL_NOT_MATCH : %s\n", url);
     return process_invalid_path_request(out);