When the fractional part in a multiplication of DECIMALs
overflowed, we truncated the first operand rather than the
longest. Now truncating least significant places instead
for more precise multiplications.

(Queuing at demand of Trudy/Davi.)

mysql-test/r/type_newdecimal.result:
  show that if we need to truncate the scale of an operand, we pick the
  right one (that is, we discard the least significant decimal places)
mysql-test/t/type_newdecimal.test:
  show that if we need to truncate the scale of an operand, we pick the
  right one (that is, we discard the least significant decimal places)
strings/decimal.c:
  when needing to disregard fractional parts, pick the least
  significant ones