python: Make two functions return gdbpy_ref<>

python: Make two functions return gdbpy_ref<>

I noticed that we release a gdbpy_ref in pretty_print_one_value only to
create it again later.  This patch fills the gap by returning a
gdbpy_ref all the way.

gdb/ChangeLog:

	* python/py-prettyprint.c (pretty_print_one_value): Return
	gdbpy_ref<>.
	(print_string_repr): Adjust.
	(apply_varobj_pretty_printer): Return gdbpy_ref<>.
	* python/python-internal.h (apply_varobj_pretty_printer): Return
	gdbpy_ref<>.
	* varobj.c (varobj_value_get_print_value): Adjust.