Cygwin: pty: Fix 'Bad address' error when running 'cmd.exe /c dir'

Cygwin: pty: Fix 'Bad address' error when running 'cmd.exe /c dir'

- If the command executed is 'cmd.exe /c [...]', runpath in spawn.cc
  will be NULL. In this case, is_console_app(runpath) check causes
  access violation. This case also the command executed is obviously
  console app., therefore, treat it as console app to fix this issue.

  Addresses: https://github.com/msys2/msys2-runtime/issues/108