C function migration from 9.2 to 9.5

Hi,

We've been having a Postgresql database with some custom C functionality
happily running for many years now. It's been running on 9.2, and we wish
to upgrade this to the latest version. However, we're seeing some issues
with the database process crashing each time.

A simplified, minimal example of the stuff that seems to get us into
trouble:

SQL definitions:
----

CREATE TYPE sessionData AS ( a int4, b int4, c int4 );

CREATE OR REPLACE FUNCTION
__WCI_SCHEMA__.getSessionData
()
RETURNS sessionData AS
'db_libdir/db_lib', 'session_get'
LANGUAGE 'C' STABLE;

----
Function definition:
----

PG_FUNCTION_INFO_V1 (session_get);
Datum session_get(PG_FUNCTION_ARGS) {
Datum ret = packSessionData( 0, 0, 0, fcinfo);
return ret;
}

Datum packSessionData( int a, int b, int c, FunctionCallInfo fcinfo )
{
TupleDesc td;
if ( get_call_result_type( fcinfo, NULL, & td ) != TYPEFUNC_COMPOSITE )
{
ereport( ERROR,
( errcode( ERRCODE_DATA_EXCEPTION ),
errmsg( "\'packSessionData\': Function returning record
called in context that cannot accept type record" ) ) );
}
td = BlessTupleDesc( td );

Datum * ret = ( Datum * ) palloc( 3 * sizeof( Datum ) );
bool isNull[ 3 ] = {false, false, false};

ret[ 0 ] = Int32GetDatum( a );
ret[ 1 ] = Int32GetDatum( b );
ret[ 2 ] = Int32GetDatum( c );

HeapTuple ht = ( HeapTuple ) heap_form_tuple( td, ret, isNull );
return HeapTupleGetDatum( ht );
}

----

Running "select getSessionData()" with this code crashes the database.
Pretty much identical code worked fine in 9.2; it's only from 9.3+ we run
into trouble. I'm sure we're overseeing something completely basic that has
changed in 9.3, but having trouble seeing what it is. A little help would
be appreciated.

Regards,

Michael A.