Fix PR remote/20398: File-IO write always outputs "Quit"

Fix PR remote/20398: File-IO write always outputs "Quit"

Commit bb7c96deb1a1 ("gdb/remote-fileio.c: Eliminate custom SIGINT
signal handler") regressed the File-IO support.

Failed output:

  (gdb) target remote :8888
  Remote debugging using :8888
  0x00008098 in _start ()
  (gdb) c
  Continuing.
  Quit
  Quit
  Quit
  Quit
  Quit
  Quit
  Quit
  Quit
  Quit
  Quit
  Quit
  [Inferior 1 (Remote target) exited normally]

Expected output:

  (gdb) target remote :8888
  Remote debugging using :8888
  0x00008098 in _start ()
  (gdb) c
  Continuing.
  i: 0
  i: 1
  i: 2
  i: 3
  i: 4
  i: 5
  i: 6
  i: 7
  i: 8
  i: 9
  [Inferior 1 (Remote target) exited normally]

The problem that the new File-IO quit handler forgets to check the
quit flag before calling throwing a quit.

gdb/ChangeLog:
2016-08-05  Pedro Alves  <palves@redhat.com>

	PR remote/20398
	* remote-fileio.c (remote_fileio_quit_handler): Check the quit
	flag before calling quit.