Problem:  When we have a really large number (between 2^63 and 2^64)
as the left side of the mod operator, it gets improperly corerced
into a signed value.

Solution:  Added check to see if the "negative" number is really
positive, and if so, cast it.


mysql-test/r/bigint.result:
  Added test case result
mysql-test/t/bigint.test:
  Added test case
sql/item_func.cc:
  Added check to see if we have an unsigned number that looks like a
  signed number.