This changes read_memory_robust to return a std::vector, allowing the
removal of free_memory_read_result_vector and associated cleanups.
This patch also changes the functions it touches to be a bit more
robust with regards to deallocation; it's perhaps possible that
read_memory_robust could have leaked in some situations.
This patch is based on my earlier series to remove some MI cleanups.
Regression tested by the buildbot.
gdb/ChangeLog
2017-09-29 Tom Tromey <tom@tromey.com>