Comment update in BuildTupleFromCStrings()

Hi,
BuildTupleFromCStrings() has comment "/* Call the "in" function for
each non-dropped attribute */". It then calls the in function even
when it's going to set that attribute to NULL.
1189         if (!TupleDescAttr(tupdesc, i)->attisdropped)
1190         {
1191             /* Non-dropped attributes */
1192             dvalues[i] = InputFunctionCall(&attinmeta->attinfuncs[i],
1193                                            values[i],
1194                                            attinmeta->attioparams[i],
1195                                            attinmeta->atttypmods[i]);
1196             if (values[i] != NULL)
1197                 nulls[i] = false;
1198             else
1199                 nulls[i] = true;
1200         }

 If we are setting isnull to true i.e. it's a NULL value, dvalues
value doesn't matter but we still invoke corresponding in function,
which looks strange and the comment doesn't help. But there's code in
make_tuple_from_result_row() which does the same thing and explain why
we need to invoke in() function even on the NULL values. I thought,
the same comment applies here. Here's patch to update the comment in
BuildTupleFromCStrings().

The code in make_tuple_from_result_row() that converts an array of
values string to tuple looks quite similar to what
BuildTupleFromCStrings() is doing with a small difference that
make_tuple_from_result_row() maintains and error context to report the
attribute whose in() function caused an error. May be we could pass an
optional error context to the later and use it.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company