Fixed crash caused by x64 int/long incompatibility introduced
in Bug #29125.


sql/item_timefunc.cc:
  Fixed crash caused by int/long incompatibility on x64 systems.
                  
  Changed two "uint" casts and a "long" declartion to "int" in order to
  ensure that the integer sign is preserved.
                  
  See Bug #48739 for details.