Input validation

In FreeBSD 12.0-STABLE before r351264, 12.0-RELEASE before 12.0-RELEASE-p10, 11.3-STABLE before r351265, 11.3-RELEASE before 11.3-RELEASE-p3, and 11.2-RELEASE before 11.2-RELEASE-p14, the kernel driver for /dev/midistat implements a read handler that is not thread-safe. A multi-threaded program can exploit races in the handler to copy out kernel memory outside the boundaries of midistat’s data buffer.