Pull up following revision(s) (requested by kamil in ticket #898):

usr.sbin/installboot/arch/i386.c: revision 1.41

Fix integer overflow in installboot(8)
Add a sanity check of the disk_buf first three bytes. The original code on
a disk with nul bytes was causing integer overflow and thus calling the
memcmp(3) functin in is_zero() with enormous length.
Verity that the 0th byte is JMP, 1th a signed byte >= 9 to prevent overflow
and 2th byte NOP.

Add a comment explaining the check.

Detected with MKSANITIZER and ASan.

(martin)