Fix bug that debugger raise an error when breakpoints are set in script snippets

[ono-max]

@ko1-san giving me the following bug report:

Describe the bug

After setting breakpoints at a script snippet in Snippets pane, execute debugger. Then debugger raise an error such as:

DEBUGGER: wait for debugger connection...
DEBUGGER: Connected.
DEBUGGER: ReaderThreadError: undefined method `[]' for nil:NilClass
["/Users/naotto/workspace/debug/lib/debug/server_cdp.rb:198:in `block in process'",
 "/Users/naotto/workspace/debug/lib/debug/server_cdp.rb:106:in `loop'",
 "/Users/naotto/workspace/debug/lib/debug/server_cdp.rb:106:in `process'",
 "/Users/naotto/workspace/debug/lib/debug/server.rb:69:in `block (3 levels) in activate'",
 "/Users/naotto/workspace/debug/lib/debug/server.rb:212:in `setup_interrupt'",
 "/Users/naotto/workspace/debug/lib/debug/server.rb:67:in `block (2 levels) in activate'",
 "/Users/naotto/workspace/debug/lib/debug/server.rb:359:in `block (2 levels) in accept'",
 "/Users/naotto/.rbenv/versions/2.6.7/lib/ruby/2.6.0/socket.rb:809:in `block (2 levels) in accept_loop'",
 "/Users/naotto/.rbenv/versions/2.6.7/lib/ruby/2.6.0/socket.rb:806:in `each'",
 "/Users/naotto/.rbenv/versions/2.6.7/lib/ruby/2.6.0/socket.rb:806:in `block in accept_loop'",
 "/Users/naotto/.rbenv/versions/2.6.7/lib/ruby/2.6.0/socket.rb:804:in `loop'",
 "/Users/naotto/.rbenv/versions/2.6.7/lib/ruby/2.6.0/socket.rb:804:in `accept_loop'",
 "/Users/naotto/workspace/debug/lib/debug/server.rb:357:in `block in accept'",
 "/Users/naotto/.rbenv/versions/2.6.7/lib/ruby/2.6.0/socket.rb:781:in `tcp_server_sockets'",
 "/Users/naotto/workspace/debug/lib/debug/server.rb:338:in `accept'",
 "/Users/naotto/workspace/debug/lib/debug/server.rb:48:in `block in activate'"]
DEBUGGER: Disconnected.

Cause

The reason why this error occurs is urls of script snippets are started with http://debuggee~. This PR fixes it.