MDEV-20495 Assertion `precision > 0' failed in decimal_bin_size upon CREATE .. SELECT with zerofilled decimal

Also fixes:

MDEV-20560 Assertion `precision > 0' failed in decimal_bin_size upon SELECT with MOD short unsigned decimal

Changing the way how Item_func_mod calculates its max_length.
It now uses decimal_precision(), decimal_scale() and unsigned_flag
of its arguments, like all other Item_num_op descendants do.