Remove call to reset from compile_to_object

Remove call to reset from compile_to_object

compile_to_object declares 'error_message' and then immediately calls
reset on it.  It seemed better to change it to use initialization
instead; and then I noticed that set_arguments could return a
unique_xmalloc_ptr<char> itself.

2021-01-23  Tom Tromey  <tom@tromey.com>

	* compile/compile-internal.h (class compile_instance)
	<set_arguments>: Change return type.
	* compile/compile.c (compile_to_object): Remove call to reset.
	(compile_instance::set_arguments): Change return type.