Fix undefined behavior in gdbserver

Fix undefined behavior in gdbserver

PR gdb/26742 points out some undefined behavior in gdbserver.  The bug
is that remove_thread does:

  free_one_thread (thread);
  if (current_thread == thread)
    current_thread = NULL;

However, the equality check is undefined, because "thread" has already
been freed.

This patch fixes the bug by moving the check earlier.

Tested on x86-64 Fedora 32.

2020-10-20  Tom Tromey  <tromey@adacore.com>

	PR gdb/26742:
	* inferiors.cc (remove_thread): Clear current_thread before
	freeing the thread.