adpcmenc: Calculate the IMA_QT predictor without overflow

Previously, the value given to put_bits was 10 bits long for positive
predictors, even though 9 bits were to be written. The extra bit could
in some cases overwrite existing bits in the bitstream writer cache.

This fixes a failed assert in put_bits.h, when running a version
built with -DDEBUG.

The fate test result gets slightly improved, thanks to getting rid
of the overwritten bits in the bitstream writer cache.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/adpcmenc.c

@@ -570,7 +570,7 @@ static int adpcm_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
         init_put_bits(&pb, dst, pkt_size * 8);
 
         for (ch = 0; ch < avctx->channels; ch++) {
-            put_bits(&pb, 9, (c->status[ch].prev_sample + 0x10000) >> 7);
+            put_bits(&pb, 9, (c->status[ch].prev_sample & 0xFFFF) >> 7);
             put_bits(&pb, 7,  c->status[ch].step_index);
             if (avctx->trellis > 0) {
                 uint8_t buf[64];

tests/ref/fate/acodec-adpcm-ima_qt

@@ -1,4 +1,4 @@
-057d27978b35888776512e4e9669a63b *tests/data/fate/acodec-adpcm-ima_qt.aiff
+23cbae1182e150ebf28e0abfb9cba127 *tests/data/fate/acodec-adpcm-ima_qt.aiff
 281252 tests/data/fate/acodec-adpcm-ima_qt.aiff
-169c40435c68d50112c9c61fc67e446d *tests/data/fate/acodec-adpcm-ima_qt.out.wav
-stddev:  918.61 PSNR: 37.07 MAXDIFF:34029 bytes:  1058400/  1058560
+b0fafd002c38fb70acaddfda1a31ed61 *tests/data/fate/acodec-adpcm-ima_qt.out.wav
+stddev:  904.76 PSNR: 37.20 MAXDIFF:34029 bytes:  1058400/  1058560