check meta pointer before calling avifMetaDestroy (#2065)

fixes a NULL dereference should avifMetaCreate() fail and
avifDecoderDataDestroy() is called to clean up.

Found by Nallocfuzz (https://github.com/catenacyber/nallocfuzz).
Reported by Philippe Antoine (p.antoine <at> catenacyber.fr).

src/read.c

diff --git a/src/read.c b/src/read.c
index 7606f1e..6d204cc 100644
--- a/src/read.c
+++ b/src/read.c
@@ -993,7 +993,9 @@
 
 static void avifDecoderDataDestroy(avifDecoderData * data)
 {
-    avifMetaDestroy(data->meta);
+    if (data->meta) {
+        avifMetaDestroy(data->meta);
+    }
     for (uint32_t i = 0; i < data->tracks.count; ++i) {
         avifTrack * track = &data->tracks.track[i];
         if (track->sampleTable) {