plperl: Correctly handle empty arrays in plperl_ref_from_pg_array.

plperl_ref_from_pg_array() didn't consider the case that postgrs arrays
can have 0 dimensions (when they're empty) and accessed the first
dimension without a check. Fix that by special casing the empty array
case.

Author: Alex Hunsaker
Reported-By: Andres Freund / valgrind / buildfarm animal skink
Discussion: 20160308063240.usnzg6bsbjrne667@alap3.anarazel.de
Backpatch: 9.1-

src/pl/plperl/plperl.c

@@ -1450,6 +1450,13 @@ plperl_ref_from_pg_array(Datum arg, Oid typid)
 	info->ndims = ARR_NDIM(ar);
 	dims = ARR_DIMS(ar);
 
+	/* No dimensions? Return an empty array */
+	if (info->ndims == 0)
+	{
+		av = newRV_noinc((SV *) newAV());
+	}
+	else
+	{
 		deconstruct_array(ar, elementtype, typlen, typbyval,
 						  typalign, &info->elements, &info->nulls,
 						  &nitems);
@@ -1461,6 +1468,7 @@ plperl_ref_from_pg_array(Datum arg, Oid typid)
 			info->nelems[i] = info->nelems[i - 1] / dims[i - 1];
 
 		av = split_array(info, 0, nitems, 0);
+	}
 
 	hv = newHV();
 	(void) hv_store(hv, "array", 5, av, 0);
@@ -1479,6 +1487,9 @@ split_array(plperl_array_info *info, int first, int last, int nest)
 	int			i;
 	AV		   *result;
 
+	/* we should only be called when we have something to split */
+	Assert(info->ndims > 0);
+
 	/* since this function recurses, it could be driven to stack overflow */
 	check_stack_depth();