avifenc: Fix y4m/stdin corner cases when trying to detect a "single image" encode

apps/avifenc.c

diff --git a/apps/avifenc.c b/apps/avifenc.c
index 1a6ab6f..19e92c0 100644
--- a/apps/avifenc.c
+++ b/apps/avifenc.c
@@ -188,6 +188,13 @@
     }
     return &input->files[input->fileIndex];
 }
+static avifBool avifInputHasRemainingData(avifInput * input)
+{
+    if (input->useStdin) {
+        return !feof(stdin);
+    }
+    return (input->fileIndex < input->filesCount);
+}
 
 static avifAppFileFormat avifInputReadImage(avifInput * input, avifImage * image, uint32_t * outDepth)
 {
@@ -777,7 +784,7 @@
     encoder->keyframeInterval = keyframeInterval;
 
     uint32_t addImageFlags = AVIF_ADD_IMAGE_FLAG_NONE;
-    if (input.filesCount == 1) {
+    if (!avifInputHasRemainingData(&input)) {
         addImageFlags |= AVIF_ADD_IMAGE_FLAG_SINGLE;
     }