Change type of encoding argument to gdbpy_extract_lazy_string

Change type of encoding argument to gdbpy_extract_lazy_string

This changes gdbpy_extract_lazy_string's "encoding" argument to be a
unique_xmalloc_ptr.  I chose this rather than std::string because it
can sometimes be NULL.

2017-01-10  Tom Tromey  <tom@tromey.com>

	* python/py-prettyprint.c (print_string_repr, print_children):
	Update.
	* python/py-lazy-string.c (gdbpy_extract_lazy_string): Change type
	of "encoding".
	* varobj.c (varobj_value_get_print_value): Update.
	* python/python-internal.h (gdbpy_extract_lazy_string): Update.