electronicarts: check size before reading duration out of a chunk.

Fixes null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/electronicarts.c

@@ -545,9 +545,11 @@ static int ea_read_packet(AVFormatContext *s,
             case AV_CODEC_ID_ADPCM_EA_R1:
             case AV_CODEC_ID_ADPCM_EA_R2:
             case AV_CODEC_ID_ADPCM_IMA_EA_EACS:
+                if (pkt->size >= 4)
                     pkt->duration = AV_RL32(pkt->data);
                 break;
             case AV_CODEC_ID_ADPCM_EA_R3:
+                if (pkt->size >= 4)
                     pkt->duration = AV_RB32(pkt->data);
                 break;
             case AV_CODEC_ID_ADPCM_IMA_EA_SEAD: