This patch fixes a discrepancy in the Go and Solidity FPVM implementations.
The set of states, wherein their pre-image offsets are at the pre-image
EOF, is considered valid. The transition for these states is a zero-byte
pre-image read.
This behavior is similar to the way Linux handles read(2) syscalls where
reading at an EOF stream yields zero bytes read.

PreimageOracle.sol is updated to accommodate this by allowing pre-image
parts to be loaded at the pre-image EOF.