Fix compiler warning in linux-namespaces.c

Fix compiler warning in linux-namespaces.c

../../gdb/nat/linux-namespaces.c: In function ‘void mnsh_main(int)’:
../../gdb/nat/linux-namespaces.c:604:8: warning: ‘fd’ may be used uninitialized in this function [-Wmaybe-uninitialized]
  close (fd);
  ~~~~~~^~~~

And the warning is correct -- mnsh_recv_message can return -1 and leave fd
uninitialized, and mnsh_main will still call close (fd) if that happens.

Initialize fd to -1 to avoid that.

gdb/ChangeLog:

2019-08-27  Christian Biesinger  <cbiesinger@google.com>

	* nat/linux-namespaces.c (mnsh_main): Initialize fd (to -1).