router: avoid out of range with empty path(#204)

Co-authored-by: Julien Schmidt <git@julienschmidt.com>

router.go

@@ -232,7 +232,7 @@ func (r *Router) DELETE(path string, handle Handle) {
 // frequently used, non-standardized or custom methods (e.g. for internal
 // communication with a proxy).
 func (r *Router) Handle(method, path string, handle Handle) {
-	if path[0] != '/' {
+	if len(path) < 1 || path[0] != '/' {
 		panic("path must begin with '/' in path '" + path + "'")
 	}