proto_read_attribute_array: When count is zero, make result a NULL ptr.

I am a bit uncertain about this change, since both NULL and non-NULL
will make test cases fail within the SoftHSM test suite for example
(some SoftHSM C_ functions want the pointer to be NULL when there
are no attributes, and some allow (expect) it to be possible to pass a
non-NULL pointer with a zero count).

It seems that making it a NULL pointer when the count is 0 is the most
sensible thing though. How could the C_ function use the data pointed
to by the pointer, when the count says there is no data there? The
result would really be undefined.

gck-rpc-dispatch.c

@@ -460,6 +460,16 @@ proto_read_attribute_array(CallState * cs, CK_ATTRIBUTE_PTR * result,
 	    (&msg->buffer, msg->parsed, &msg->parsed, &n_attrs))
 		return PARSE_ERROR;
 
+	if (! n_attrs) {
+		/* If there are no attributes, it makes most sense to make result
+		 * a NULL pointer. What use could one have of a potentially dangling
+		 * pointer anyways?
+		 */
+		*result = NULL_PTR;
+		*n_result = n_attrs;
+		return CKR_OK;
+	}
+
 	/* Allocate memory for the attribute structures */
 	attrs = call_alloc(cs, n_attrs * sizeof(CK_ATTRIBUTE));
 	if (!attrs)