debugger: fix unhandled error in setBreakpoint

Fix Interface.setBreakpoint() to correctly handle an attempt to set a
breakpoint in the current script when there is no current script.
This usually happens when the debugged process is not paused.

Fixes: #6453
PR-URL: #6460
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>

lib/_debugger.js

@@ -1360,6 +1360,12 @@ Interface.prototype.setBreakpoint = function(script, line,
     script = this.client.currentScript;
   }
 
+  if (script === undefined) {
+    this.print('Cannot determine the current script, ' +
+        'make sure the debugged process is paused.');
+    return;
+  }
+
   if (/\(\)$/.test(script)) {
     // setBreakpoint('functionname()');
     var req = {