PR gdb/17210 - fix possible memory leak in read_memory_robust

PR gdb/17210 concerns a possible memory leak in read_memory_robust.
The bug can happen because read_memory_robust allocates memory, does
not install any cleanups, and invokes QUIT.  Similarly, target_read
calls QUIT, so it too can potentially throw.

The fix is to install cleanups to guard the allocated memory.

Built and regtested on x86-64 Fedora 23.  I couldn't think of a way to
test this, so no new test; and of course this means it should have
more careful review.

2016-06-29  Tom Tromey  <tom@tromey.com>

	PR gdb/17210:
	* target.c (free_memory_read_result_vector): Take a pointer to the
	VEC as an argument.
	(read_memory_robust): Install a cleanup for "result".
	* mi/mi-main.c (mi_cmd_data_read_memory_bytes): Update.