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: https://github.com/joyent/node/issues/6453
PR-URL: https://github.com/joyent/node/pull/6460
Reviewed-By: Chris Dickinson <christopher.s.dickinson@gmail.com>