server: fix crash in case of requests with nil URL (#458)

---------

Co-authored-by: aler9 <46489434+aler9@users.noreply.github.com>

server_conn.go

@@ -208,10 +208,15 @@ func (sc *ServerConn) handleRequestInner(req *base.Request) (*base.Response, err
 	if cseq, ok := req.Header["CSeq"]; !ok || len(cseq) != 1 {
 		return &base.Response{
 			StatusCode: base.StatusBadRequest,
-			Header:     base.Header{},
 		}, liberrors.ErrServerCSeqMissing{}
 	}
 
+	if req.Method != base.Options && req.URL == nil {
+		return &base.Response{
+			StatusCode: base.StatusBadRequest,
+		}, liberrors.ErrServerInvalidPath{}
+	}
+
 	sxID := getSessionID(req.Header)
 
 	var path string