apedec: prevent out of array writes in decode_array_0000

s->decoded_buffer is allocated with a min_size of:
    2 * FFALIGN(blockstodecode, 8) * sizeof(*s->decoded_buffer)

Then it is assigned to s->decoded[0] (and s->decoded_buffer + FFALIGN(blockstodecode, 8)
to s->decoded[1]) and passed as out buffer to decode_array_0000.

In this function 64 elements of the out buffer are written
unconditionally and outside the array if blockstodecode is too small.

This causes memory corruption, leading to segmentation faults or other
crashes.

Thus change decode_array_0000 to write at most blockstodecode elements
of the out buffer.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/apedec.c

@@ -592,14 +592,14 @@ static void decode_array_0000(APEContext *ctx, GetBitContext *gb,
     int ksummax, ksummin;
 
     rice->ksum = 0;
-    for (i = 0; i < 5; i++) {
+    for (i = 0; i < FFMIN(blockstodecode, 5); i++) {
         out[i] = get_rice_ook(&ctx->gb, 10);
         rice->ksum += out[i];
     }
     rice->k = av_log2(rice->ksum / 10) + 1;
     if (rice->k >= 24)
         return;
-    for (; i < 64; i++) {
+    for (; i < FFMIN(blockstodecode, 64); i++) {
         out[i] = get_rice_ook(&ctx->gb, rice->k);
         rice->ksum += out[i];
         rice->k = av_log2(rice->ksum / ((i + 1) * 2)) + 1;