gdb/sim: Assert that the simulator ptid is not null_ptid

We assign the simulator inferior a fake ptid.  If this ptid is ever
set to null_ptid then we are going to run into problems - the
simulator ptid is what we return from gdbsim_target::wait, and this in
turn is used to look up the inferior data with a call to
find_inferior_pid, which asserts the pid is not 0 (which it is in
null_pid).

This commit adds an assert that the simulator's fake pid is not
null_ptid.  There should be no user visible changes after this commit.

gdb/ChangeLog:

	* remote-sim.c (sim_inferior_data::sim_inferior_data): Assert that
	we don't set the fake simulator ptid to the null_ptid.

Change-Id: I6e08effe70e70855aea13c9caf4fd6913d5af56d