fixed builder_cancel macro to return NULL on failed build

src/libstrongswan/credentials/builder.c

@@ -37,3 +37,12 @@ ENUM(builder_part_names, BUILD_FROM_FILE, BUILD_END,
 	"BUILD_X509_FLAG",
 	"BUILD_END",
 );
+
+/**
+ * See header.
+ */
+void* builder_free(builder_t *this)
+{
+	free(this);
+	return NULL;
+}