Remove cleanups from break-catch-syscall.c

This removes the remaining cleanups from break-catch-syscall.c by
storing temporary strings in a vector.

ChangeLog
2017-10-19  Tom Tromey  <tom@tromey.com>

	* break-catch-syscall.c (catch_syscall_completer): Use
	std::string, gdb::unique_xmalloc_ptr.