[binutils-gdb] Use std::vector in find_instruction_backward

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT 52d214d3e1b2f6a1382feafbf2984acdb24c0c95 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 52d214d3e1b2f6a1382feafbf2984acdb24c0c95

Use std::vector in find_instruction_backward

This changes find_instruction_backward to use std::vector, removing a
cleanup.

gdb/ChangeLog
2017-04-12  Tom Tromey  <tom@tromey.com>

	* printcmd.c (find_instruction_backward): Use std::vector.