Disallow setting $sp < $ssp using CFS and CFSI instructions

[Dentosal]

This is a critical security bug that allows bypassing memory ownership protection completely.

Exploiting this bug is trivial. Using the following code would make the whole VM memory writable using e.g. SB and SW instructions. This in turn allows for instance performing contract operations using different contracts id.

cfs $sp           // Use the bug to set $sp = 0
slli $a, $one, 26 // Set $a to VM_MAX_RAM
aloc $a           // Set $hp = 0, so that the whole memory is in owned heap range

[xgreenx]

Good catch!=) Could you also update the CHANELOG please?=)