Comparison of a BIGINT NOT NULL column with a constant arithmetic
expression that evaluates to NULL caused error 1048: "Column '...'
cannot be null".

Made convert_constant_item() check if the constant expression is NULL
before attempting to store it in a field. Attempts to store NULL in a
NOT NULL field caused query errors.


sql/item_cmpfunc.cc:
  Fixed bug #32335.
  1. Made convert_constant_item() check if the constant expression is NULL
     before attempting to store it in a field. Attempts to store NULL in
     a NOT NULL field caused query errors.
  
  2. Also minor bug has been fixed: the thd->count_cuted_fields value
     was not restored in case of successful conversion.
mysql-test/t/select.test:
  Added test case for bug #32335.
mysql-test/r/select.result:
  Added test case for bug #32335.