avcodec/apedec: Fix integer overflows in predictor_decode_mono_3950()

Fixes: signed integer overflow: -2147407150 + -1871606 cannot be represented in type 'int'
Fixes: 18702/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_APE_fuzzer-5679095417667584

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/apedec.c

@@ -1208,14 +1208,14 @@ static void predictor_decode_mono_3950(APEContext *ctx, int count)
         A = *decoded0;
 
         p->buf[YDELAYA] = currentA;
-        p->buf[YDELAYA - 1] = p->buf[YDELAYA] - p->buf[YDELAYA - 1];
+        p->buf[YDELAYA - 1] = p->buf[YDELAYA] - (unsigned)p->buf[YDELAYA - 1];
 
         predictionA = p->buf[YDELAYA    ] * p->coeffsA[0][0] +
                       p->buf[YDELAYA - 1] * p->coeffsA[0][1] +
                       p->buf[YDELAYA - 2] * p->coeffsA[0][2] +
                       p->buf[YDELAYA - 3] * p->coeffsA[0][3];
 
-        currentA = A + (predictionA >> 10);
+        currentA = A + (unsigned)(predictionA >> 10);
 
         p->buf[YADAPTCOEFFSA]     = APESIGN(p->buf[YDELAYA    ]);
         p->buf[YADAPTCOEFFSA - 1] = APESIGN(p->buf[YDELAYA - 1]);