Fix follow-fork latent bug

Fix follow-fork latent bug

A later patch in the series adds an assertion to switch_to_thread that
the resulting inferior_ptid always matches the "current_inferior()"
inferior.  This exposed a latent bug in the follow-fork code, where
we're building the fork child inferior.  We're switching
inferior_ptid, but not the current inferior object...

gdb/ChangeLog:
2017-04-13  Pedro Alves  <palves@redhat.com>

	* infrun.c (follow_fork_inferior): Also switch the current
	inferior.