avformat/mov: Fix nullptr dereference with invalid encryption metadata.

Found by fuzzer.

Bug: https://crbug.com/356720789
Signed-off-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -8143,15 +8143,19 @@ static int cenc_filter(MOVContext *mov, AVStream* st, MOVStreamContext *sc, AVPa
             return AVERROR_INVALIDDATA;
         }
 
+        encrypted_sample = NULL;
         if (!encryption_index->nb_encrypted_samples) {
             // Full-sample encryption with default settings.
             encrypted_sample = sc->cenc.default_encrypted_sample;
         } else if (encrypted_index >= 0 && encrypted_index < encryption_index->nb_encrypted_samples) {
             // Per-sample setting override.
             encrypted_sample = encryption_index->encrypted_samples[encrypted_index];
-            if (!encrypted_sample)
+            if (!encrypted_sample) {
                 encrypted_sample = sc->cenc.default_encrypted_sample;
-        } else {
+            }
+        }
+
+        if (!encrypted_sample) {
             av_log(mov->fc, AV_LOG_ERROR, "Incorrect number of samples in encryption info\n");
             return AVERROR_INVALIDDATA;
         }