sipvoiplink: check find result before substr

Change-Id: Icca68a88a43b351b0bc5822888703ba774ee0138

src/sip/sipvoiplink.cpp

@@ -227,14 +227,13 @@ transaction_request_cb(pjsip_rx_data* rdata)
 
         if (request.find("NOTIFY") != std::string_view::npos) {
             if (body and body->data) {
+                std::string_view body_view(static_cast<char*>(body->data), body->len);
+                auto pos = body_view.find("Voice-Message: ");
+                if (pos != std::string_view::npos) {
                     int newCount {0};
                     int oldCount {0};
                     int urgentCount {0};
-
-                std::string sp(static_cast<char*>(body->data), body->len);
-                auto pos = sp.find("Voice-Message: ");
-                sp = sp.substr(pos);
-
+                    std::string sp(body_view.substr(pos));
                     int ret = sscanf(sp.c_str(),
                                     "Voice-Message: %d/%d (%d/",
                                     &newCount,
@@ -249,6 +248,7 @@ transaction_request_cb(pjsip_rx_data* rdata)
                                                                        oldCount,
                                                                        urgentCount);
                 }
+            }
         } else if (request.find("MESSAGE") != std::string_view::npos) {
             // Reply 200 immediately (RFC 3428, ch. 7)
             try_respond_stateless(endpt_, rdata, PJSIP_SC_OK, nullptr, nullptr, nullptr);