fixed SQLProcessorSelect::getTableInfoFromVarName()

- removed static qualifier, because it is no longer used
  form static functions.
- removed SQLSelectInfo from the argument list. By the
  above fix, this function can access to the private context.
- return NULL when the table information is not found
  instead of throwing SQLProcessorException. We will add
  code to SQLProcessorException in getTableInfoFromColumnInfo().