Fix handling of NULL values retrieved by INT8 columns.

The former coding failed to handle NULL values correctly when
retrieved from Informix. This caused a conversion error where it just
have to emit a NULL datum into the tuple slot. While at it, make
the INT8/BIGINT conversion routines smarter about invalid string
buffers when converting a INT8/BIGINT value into its string
representation by marking the corresponding attribute as
INDICATOR_NOT_VALID.