Before, the row value was maximized against _numlines already, but the
value from _numlines is not limited anywhere, so it could be longer than
the length of _row_offsets. This check makes sure the array bounds is
never exceeded.
Since these are memory addresses, there is no need to make them signed.
Furthermore, the HD44780 chip supports memory addresses up to 0x67, so
uint8_t shouldbe sufficient.
