Fixed uninitialised read UB in Argon2

Fixed uninitialised read UB in Argon2

The index block was declared in the block loop instead of the segment
loop.  Yet it's only initialised one time out of 128 there, so most of
the time we're accessing uninitialised memory.

It still appeared to work because that that block always occupied the
same spot in the stack. Only Clang's memory sanitiser and the TIS
interpreter caught this.