gdbinit: use ____print_str to print htable keys

I noticed this problem while dumping the contents of EG(function_table),
where keys for closures start with a null byte. printf interprets this
as a zero-length string and emits nothing. This allows the key to be
rendered properly in readable form.

Closes GH-6577.

.gdbinit

@@ -352,7 +352,8 @@ define ____print_ht
 			end
 			printf "[%d] ", $i
 			if $p->key
-				printf "%s => ", $p->key->val
+				____print_str $p->key->val $p->key->len
+				printf " => "
 			else
 				printf "%d => ", $p->h
 			end