gdbserver: Don't add extra NULL to program args

The vector holding the program args is passed as a parameter
to target_create_inferior, which then passes it to
stringify_argv for all platforms, where any NULL entry in
the vector is ignored, so there seems to be no reason
to actually add one after all.

(Since the intention is to replace uses of stringify_argv with
construct_inferior_arguments in a follow-up commit and that
function doesn't currently handle such NULL arguments, it
would otherwise have to be extended.)

gdbserver/ChangeLog:

	* server.cc (captured_main), (handle_v_run): No longer
	insert extra NULL element to args vector.

Change-Id: Ia2ef6d36814a6b11ce8b0d6e3b33248a7945e825