Hoist assertion in target_stack::unpush

Hoist assertion in target_stack::unpush

I noticed that target_stack::unpush first uses the target "t", then
later asserts that it is non-NULL:

  strata stratum = t->stratum ();
[...]
  gdb_assert (t != NULL);

This is backwards, though, as the assertion must come first.

gdb/ChangeLog
2019-02-05  Tom Tromey  <tom@tromey.com>

	* target.c (target_stack::unpush): Move assertion earlier.