snippet/uncompng: fix encode_apng return value

snippet/uncompng.c

diff --git a/snippet/uncompng.c b/snippet/uncompng.c
index 4406675..c4a8ae4 100644
--- a/snippet/uncompng.c
+++ b/snippet/uncompng.c
@@ -856,7 +856,7 @@
   int ret = (*write_func)(context, uncompng__private_impl_buffer, (size_t)ej);
 
   concurrent = false;
-  return 0;
+  return ret;
 }
 
 UNCOMPNG__MAYBE_STATIC int  //
@@ -922,7 +922,7 @@
   }
 
   concurrent = false;
-  return 0;
+  return ret;
 }
 
 #endif  // UNCOMPNG_IMPLEMENTATION