Avoid passing nil path to File.basename

[st0012]

Some frames may not aways have path value.

irb((run)@bin/rails:in `require' (not under control)):003:0> frame.location
=> "bin/rails:in `require'"
irb((run)@bin/rails:in `require' (not under control)):004:0> frame.path
=> nil

And when that happens, File.basename(nil) would cause exception and crash the debugger.

DEBUGGER: Connected.
["DEBUGGER Exception: /Users/st0012/projects/debug/lib/debug/thread_client.rb:1038",
 #<TypeError: no implicit conversion of nil into String>,
 ["/Users/st0012/projects/debug/lib/debug/server_dap.rb:623:in `basename'",
  "/Users/st0012/projects/debug/lib/debug/server_dap.rb:623:in `block in process_dap'",
  "/Users/st0012/projects/debug/lib/debug/server_dap.rb:611:in `map'",
  "/Users/st0012/projects/debug/lib/debug/server_dap.rb:611:in `process_dap'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:1027:in `wait_next_action_'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:727:in `wait_next_action'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:284:in `suspend'",
  "/Users/st0012/projects/debug/lib/debug/thread_client.rb:218:in `on_breakpoint'",
  "/Users/st0012/projects/debug/lib/debug/breakpoint.rb:69:in `suspend'",
  "/Users/st0012/projects/debug/lib/debug/breakpoint.rb:161:in `block in setup'",
  "/Users/st0012/projects/debug/lib/debug/session.rb:2002:in `setup_initial_suspend'",
  "/Users/st0012/projects/debug/lib/debug/session.rb:1992:in `open_unix'",
  "/Users/st0012/projects/debug/lib/debug/session.rb:1965:in `open'",
  "/Users/st0012/projects/debug/lib/debug/open.rb:13:in `<top (required)>'",
  "bin/rails:in `require'"]]
/Users/st0012/projects/debug/lib/debug/server_dap.rb:623:in `basename': no implicit conversion of nil into String (TypeError)
        from /Users/st0012/projects/debug/lib/debug/server_dap.rb:623:in `block in process_dap'
        from /Users/st0012/projects/debug/lib/debug/server_dap.rb:611:in `map'
        from /Users/st0012/projects/debug/lib/debug/server_dap.rb:611:in `process_dap'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:1027:in `wait_next_action_'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:727:in `wait_next_action'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:284:in `suspend'
        from /Users/st0012/projects/debug/lib/debug/thread_client.rb:218:in `on_breakpoint'
        from /Users/st0012/projects/debug/lib/debug/breakpoint.rb:69:in `suspend'
        from /Users/st0012/projects/debug/lib/debug/breakpoint.rb:161:in `block in setup'
        from /Users/st0012/projects/debug/lib/debug/session.rb:2002:in `setup_initial_suspend'
        from /Users/st0012/projects/debug/lib/debug/session.rb:1992:in `open_unix'
        from /Users/st0012/projects/debug/lib/debug/session.rb:1965:in `open'
        from /Users/st0012/projects/debug/lib/debug/open.rb:13:in `<top (required)>'
        from bin/rails:in `require'
DEBUGGER: Disconnected.

I can reproduce this with: rdbg --command --open --stop-at-load -- bundle exec rails s on any Rails apps.

(I accidentally added the unnecessary --stop-at-load and found the issue)

But of course, there may be a better solution than this patch.

[st0012]

Actually, I think frame.location.to_s is better. Will update.