Fix corner case in partial script url matching

runtime/vm/object.cc

Index: runtime/vm/object.cc
===================================================================
--- runtime/vm/object.cc	(revision 40736)
+++ runtime/vm/object.cc	(working copy)
@@ -9112,10 +9112,13 @@
 // TODO(hausner): we might want to add a script dictionary to the
 // library class to make this lookup faster.
 RawScript* Library::LookupScript(const String& url) const {
+  const intptr_t url_length = url.Length();
+  if (url_length == 0) {
+    return Script::null();
+  }
   const Array& scripts = Array::Handle(LoadedScripts());
   Script& script = Script::Handle();
   String& script_url = String::Handle();
-  const intptr_t url_length = url.Length();
   const intptr_t num_scripts = scripts.Length();
   for (int i = 0; i < num_scripts; i++) {
     script ^= scripts.At(i);