Convert Datum* to char*

Hi,

I want to read an attribute value from a TupleTableSlot. When I try to
convert an attribute of SQL type varchar from Datum* to char* with the help
of the method TextDatumGetCString(...), sometimes there is a segmentation
fault. The segmentation fault comes from the method
TextDatumGetCString(...), which is defined in utils/builtins.h.
Unfortunately, the fault is not always reproducible. I debugged the code
and figured out that the value of result->tts_values[i] sometimes is
random. It may be uninitialized memory. In other cases, the variable value
is NULL. Then, I can just skip the conversion from Datum* to char*, so that
there is no segmentation fault. I attached a patch with the code. The
relevant line is:
char *value = TextDatumGetCString(result->tts_values[i]);
The SQL-Query is a simple "SELECT * from ..." on the TPC-H table customer.
About every third execution leads to a segmentation fault.

Why is the memory of the variable uninitialized?
I am not very familiar with Postgres. Is there another method to get a
varchar attribute out of a TupleTableSlot as string?

Best regards
Maria