Fix --type-id-style hash for empty internal names.

Fix --type-id-style hash for empty internal names.

libabigail's intern_string class treats the empty string specially. It
is not safe to call the raw method without checking for a empty
pointer. It is safe to convert to std::string.

This commit changes the XML writer to convert interned strings to
std::strings before computing their hashes.

	* src/abg-writer.cc (write_context::get_id_for_type): When
	hashing internal type names, convert to std::string rather
	than using the raw method directly as this will avoid a null
	pointer dereference in the case of an empty string; tabify
	code indentation.

Signed-off-by: Giuliano Procida <gprocida@google.com>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>