UINT16_MAX would be just over a disk block, and allocating another
3.93k for a single additional segment does not make sense.

	(gdb) p /x sizeof(struct fellow_disk_seg) * 65534 +
		sizeof(struct fellow_disk_seglist)
	$7 = 0x37ffd0

Also fix a potential overflow, where a uint16_t was incremented after
clamping to UINT16_MAX.