avformat/adxdec: dont return random data on EOF

Fixes use of uninitialized memory
Partly fixes; msan_uninit-mem_7fb7d24780d0_2744_R03T.CAK
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/adxdec.c

@@ -157,9 +157,11 @@ static int adx_decode_frame(AVCodecContext *avctx, void *data,
             buf_size -= BLOCK_SIZE;
             buf      += BLOCK_SIZE;
         }
-        samples_offset += BLOCK_SAMPLES;
+        if (!c->eof)
+            samples_offset += BLOCK_SAMPLES;
     }
 
+    frame->nb_samples = samples_offset;
     *got_frame_ptr = 1;
 
     return buf - avpkt->data;