Fix some compiler warnings on older compilers

Some older compilers appear to not understand the recently introduced
PG_FINALLY code structure that well in some circumstances and complain
about possibly uninitialized variables.  So to fix, initialize the
variables explicitly in the cases complained about.

Discussion: https://www.postgresql.org/message-id/flat/95a822c3-728b-af0e-d7e5-71890507ae0c%402ndquadrant.com

src/backend/utils/adt/xml.c

@@ -3821,7 +3821,7 @@ SPI_sql_row_to_xmlelement(uint64 rownum, StringInfo result, char *tablename,
 static text *
 xml_xmlnodetoxmltype(xmlNodePtr cur, PgXmlErrorContext *xmlerrcxt)
 {
-	xmltype    *result;
+	xmltype    *result = NULL;
 
 	if (cur->type != XML_ATTRIBUTE_NODE && cur->type != XML_TEXT_NODE)
 	{

src/pl/plperl/plperl.c

@@ -1840,7 +1840,7 @@ PG_FUNCTION_INFO_V1(plperl_call_handler);
 Datum
 plperl_call_handler(PG_FUNCTION_ARGS)
 {
-	Datum		retval;
+	Datum		retval = (Datum) 0;
 	plperl_call_data *volatile save_call_data = current_call_data;
 	plperl_interp_desc *volatile oldinterp = plperl_active_interp;
 	plperl_call_data this_call_data;

src/pl/plpython/plpy_exec.c

@@ -1025,7 +1025,7 @@ plpython_trigger_error_callback(void *arg)
 static PyObject *
 PLy_procedure_call(PLyProcedure *proc, const char *kargs, PyObject *vargs)
 {
-	PyObject   *rv;
+	PyObject   *rv = NULL;
 	int volatile save_subxact_level = list_length(explicit_subtransactions);
 
 	PyDict_SetItemString(proc->globals, kargs, vargs);

src/pl/plpython/plpy_typeio.c

@@ -901,7 +901,7 @@ PLyObject_ToBytea(PLyObToDatum *arg, PyObject *plrv,
 				  bool *isnull, bool inarray)
 {
 	PyObject   *volatile plrv_so = NULL;
-	Datum		rv;
+	Datum		rv = (Datum) 0;
 
 	if (plrv == Py_None)
 	{

src/pl/tcl/pltcl.c

@@ -718,7 +718,7 @@ pltclu_call_handler(PG_FUNCTION_ARGS)
 static Datum
 pltcl_handler(PG_FUNCTION_ARGS, bool pltrusted)
 {
-	Datum		retval;
+	Datum		retval = (Datum) 0;
 	pltcl_call_state current_call_state;
 	pltcl_call_state *save_call_state;