readLongBe
Reads a 64-bit signed integer in big-endian order starting at offset.
Big-endian means the most significant byte is stored at the lowest index: the byte at offset contributes bits 56..63 and the byte at offset + 7 contributes bits 0..7.
Return
the decoded value.
Parameters
offset
the index of the first byte to read.
Throws
if offset is negative or offset + 8 exceeds the array size.