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.

Close #6453

@bnoordhuis Please review. Is it worth adding a unit test for this change? It's more than just adding few lines to `test/simple/test-debugger-repl.js`, a new test fixture is needed.