Fix negative ftell in avif_example_decode_memory (#2772)

examples/avif_example_decode_memory.c

diff --git a/examples/avif_example_decode_memory.c b/examples/avif_example_decode_memory.c
index 139f151..0a15018 100644
--- a/examples/avif_example_decode_memory.c
+++ b/examples/avif_example_decode_memory.c
@@ -39,6 +39,7 @@
     long fileSize = ftell(f);
     if (fileSize < 0) {
         fprintf(stderr, "Truncated file: %s\n", inputFilename);
+        goto cleanup;
     }
     fseek(f, 0, SEEK_SET);
     fileBuffer = malloc(fileSize);