Losing memory references - SRF + SPI

I am writing a function that returns a set of tuples by using also the
PostGIS. Thuis, I am using SRF too. It successfully returns the expected
result when it has at most 4 tuples. However, this is not the case when
more than 4 tuples have to be returned. When I debug the code, I found that
the problem is in my function that transforms a cstring after a
SPI_connection. It seems that this cstring is not valid anymore in the
moment of this conversion (see my comment below). I know that the SPI uses
different contexts when it init and finish its process. But, I don't
understand why I have this problem here. Please, note that I tried to copy
the values of the whole tuple, but I have the same problem: system crash
after the forth call of the function. Also note that I call this function
only in the init call of the SRF. Please I would appreciate any suggestion
and help.

----------- code of the problematic function here ---------------

LWGEOM *retrieve_geom_from_postgis(int row_id) {
char query[100];
int err;
char *wkt;
int srid;
LWGEOM *lwgeom = NULL;
HeapTuple cop;
bool null;
TupleDesc tupdesc;

//refin is a prepared select command that returns 2 columns
sprintf(query, "EXECUTE refinplan(%d);", row_id);

if (SPI_OK_CONNECT != SPI_connect()) {
SPI_finish();
_DEBUG(ERROR, "retrieve_geom_from_postgis: could not connect to SPI
manager");
return NULL;
}
err = SPI_execute(query, false, 1);
if (err < 0) {
SPI_finish();
_DEBUG(ERROR, "retrieve_geom_from_postgis: could not execute the
EXECUTE command");
return NULL;
}

if (SPI_processed <= 0) {
SPI_finish();
_DEBUGF(ERROR, "the row_id (%d) does not exist in the table",
row_id)
return NULL;
}
cop = SPI_copytuple(SPI_tuptable->vals[0]);
tupdesc = SPI_tuptable->tupdesc;

/* disconnect from SPI */
SPI_finish();

wkt = text2cstring(DatumGetTextP(heap_getattr(cop, 1, tupdesc, &null)));
srid = DatumGetInt32(heap_getattr(cop, 2, tupdesc, &null));

lwgeom = lwgeom_from_wkt(wkt, LW_PARSER_CHECK_NONE); //error here...
only after the forth call
lwgeom_set_srid(lwgeom, srid);

lwfree(wkt);

return lwgeom;
}