movenc: Set a correct packet size for AMR-NB mode 15, "no data"

These packets are valid packets, and consist of 1 byte (which
contains the mode bits).

This had been analyzed and reported by Igor Levin, igor d levin comverse com.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -2004,7 +2004,7 @@ int ff_mov_write_packet(AVFormatContext *s, AVPacket *pkt)
     if (enc->codec_id == CODEC_ID_AMR_NB) {
         /* We must find out how many AMR blocks there are in one packet */
         static uint16_t packed_size[16] =
-            {13, 14, 16, 18, 20, 21, 27, 32, 6, 0, 0, 0, 0, 0, 0, 0};
+            {13, 14, 16, 18, 20, 21, 27, 32, 6, 0, 0, 0, 0, 0, 0, 1};
         int len = 0;
 
         while (len < size && samplesInChunk < 100) {