Fix leak in linespec.c

Fix leak in linespec.c

Valgrind reports a leak in many tests, such as:
==9382== 16 bytes in 1 blocks are definitely lost in loss record 236 of 3,282
==9382==    at 0x4C2BE6D: malloc (vg_replace_malloc.c:309)
==9382==    by 0x4197AF: xrealloc (common-utils.c:64)
==9382==    by 0x51D16A: xresizevec<linespec_canonical_name> (poison.h:170)
==9382==    by 0x51D16A: add_sal_to_sals(linespec_state*, std::vector<symtab_and_line, std::allocator<symtab_and_line> >*, symtab_and_line*, char const*, int) (linespec.c:1041)
==9382==    by 0x51E2BF: create_sals_line_offset (linespec.c:2215)
==9382==    by 0x51E2BF: convert_linespec_to_sals(linespec_state*, linespec*) (linespec.c:2358)
==9382==    by 0x521B5D: convert_explicit_location_to_sals (linespec.c:2473)

Fix leak by xfree-ing self->canonical_names in linespec_state_destructor.
The leak probably appeared with the patch 'Remove cleanup from linespec.c',
as there was a cleanup to xfree canonical_names before the patch.

Tested on Debian/amd64, native and under valgrind.

2019-01-09  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* linespec.c (linespec_state_destructor): Free self->canonical_names.