Don't return stale data from fbsd_pid_to_exec_file for kernel processes.

Don't return stale data from fbsd_pid_to_exec_file for kernel processes.

For processes without an associated executable (such as kernel processes),
the kern.proc.pathname.<pid> system control node returns a length of zero
without modifying the user's buffer.  Detect this case and return NULL
rather than the previous contents of the static buffer 'buf'.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_pid_to_exec_file) [KERN_PROC_PATHNAME]: Return
	NULL for an empty pathname.