Fix leak in event-top.c history expansion

Fix leak in event-top.c history expansion

E.g. in gdb.base/default.exp, valgrind detects leaks such as
==17663== 1,438 bytes in 101 blocks are definitely lost in loss record 2,804 of 2,884
==17663==    at 0x4C2BE6D: malloc (vg_replace_malloc.c:309)
==17663==    by 0x418A17: xmalloc (common-utils.c:44)
==17663==    by 0x4E6F19C: history_expand (histexpand.c:1061)
==17663==    by 0x4B4490: handle_line_of_input(buffer*, char const*, int, char const*) (event-top.c:685)
==17663==    by 0x4B4562: command_line_handler(std::unique_ptr<char, gdb::xfree_deleter<char> >&&) (event-top.c:753)
...

Fix the leak by using an unique_xmalloc_ptr for history_value.

gdb/ChangeLog
2019-01-22  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* event-top.c (handle_line_of_input): use unique_xmalloc_ptr for
	history_value.