gdb: make struct linespect contain vectors, not pointers to vectors

gdb: make struct linespect contain vectors, not pointers to vectors

struct linespec contains pointers to vectors, instead of containing
vectors directly.  This is probably historical, when linespec_parser
(which contains a struct linespec field) was not C++-ified yet.  But it
seems easy to change the pointers to vectors to just vectors today.
This simplifies the code, we don't need to manually allocate and delete
the vectors and there's no pointer that can be NULL.

As far as I understand, there was not meaningful distinction between a
NULL pointer to vector and an empty vector.  So all NULL checks are
changed for !empty checks.

Change-Id: Ie759707da14d9d984169b93233343a86e2de9ee6