Add no-op pb_release() define when PB_ENABLE_MALLOC is not used
diff --git a/pb_decode.h b/pb_decode.h
index 9b70c8b..824acd4 100644
--- a/pb_decode.h
+++ b/pb_decode.h
@@ -113,6 +113,9 @@
  * pb_decode() returns with an error, the message is already released.
  */
 void pb_release(const pb_msgdesc_t *fields, void *dest_struct);
+#else
+/* Allocation is not supported, so release is no-op */
+#define pb_release(fields, dest_struct) PB_UNUSED(fields); PB_UNUSED(dest_struct);
 #endif