Fix(returndatacopy): Data offset out of bounds is still an error even if len==0

[birchmd]

I made a mistake in #26 by adding the early return when len == 0. The function is still supposed to return an error if the data_offset is out of bounds even if when len is zero. This was causing a failure in the st_memory EVM test.

I have modified the logic to simply assign memory_offset as 0 if len == 0 instead of existing the function early. This assignment is safe because memory_offset is not actually used in any of the memory-related functions in the case len is zero.

All the EVM tests now once again pass.