Fix offset condition

[eonu]

is/is not should not be used for integer comparisons.

/Users/eonu/dev/python-ibmdbsa/ibm_db_sa/base.py:411: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if offset is not 0:
/Users/eonu/dev/python-ibmdbsa/ibm_db_sa/base.py:413: SyntaxWarning: "is not" with a literal. Did you mean "!="?
  if offset is not 0 and limit is not None: