Don't call "detach_inferior" on "remote_follow_fork"

Don't call "detach_inferior" on "remote_follow_fork"

This patch fixes a regression that has been introduced by:

  commit bc09b0c14fb713a9aec25e09b78499f3bc2441b5
  Date:   Fri Jan 19 11:48:11 2018 -0500

      Make linux_nat_detach/thread_db_detach use the inferior parameter

It is possible to trigger this failure with gdb.base/foll-fork.exp (in
which case a bunch of ERROR's will be printed), but one can also use
the test below.

Consider the following example program:

  #include <unistd.h>

  int
  main (int argc, char *argv[])
  {
    fork ();

    return 0;
  }

When running it under gdbserver:

  # ./gdb/gdbserver/gdbserver --multi --once :2345

And debugging it under GDB, we see a segmentation fault:

  # ./gdb/gdb -q -batch -ex 'set remote exec-file ./a.out' -ex 'tar extended-remote :2345' -ex r ./a.out
  Starting program:
  ...
  [Detaching after fork from child process 16102.]
  Segmentation fault (core dumped)

The problem happens on inferior.c:detach_inferior:

  void
  detach_inferior (inferior *inf)
  {
    /* Save the pid, since exit_inferior_1 will reset it.  */
    int pid = inf->pid;
              ^^^^^^^^^

    exit_inferior_1 (inf, 0);

    if (print_inferior_events)
      printf_unfiltered (_("[Inferior %d detached]\n"), pid);
  }

When this code is called from remote.c:remote_follow_fork, the PID is
valid but there is no 'inferior' associated with it, which means that
'inf == NULL'.

The proper fix here is to not call "detach_inferior" when doing remote
follow-fork, because we don't have an inferior to detach on the host
side.

Before bc09b0c1, that call was already a nop (exit_inferior_1 bails
out early if you pass it a NULL inferior), except that it printed
"Inferior PID detached" when "set print inferior-events" is on.  Since
native debugging doesn't call detach_inferior in this case, removing
the call from remote aligns remote debugging output with native
debugging output further.

This has been regtested using BuildBot and no regressions were found.

gdb/ChangeLog:
2018-01-29  Sergio Durigan Junior  <sergiodj@redhat.com>

	* remote.c (remote_follow_fork): Don't call "detach_inferior".