49b4c5f864
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.