Set offsetBytesA for AVIF_RGB_FORMAT_GRAY (#3139)

src/reformat.c

diff --git a/src/reformat.c b/src/reformat.c
index 7b53add..2fd8d32 100644
--- a/src/reformat.c
+++ b/src/reformat.c
@@ -93,6 +93,7 @@
             break;
         case AVIF_RGB_FORMAT_GRAY:
             info->offsetBytesGray = info->channelBytes * 0;
+            info->offsetBytesA = 0;
             break;
         case AVIF_RGB_FORMAT_GRAYA:
             info->offsetBytesGray = info->channelBytes * 0;