avcodec/pixlet: fixes integer overflow in read_highpass()

Fixes: runtime error: negation of -2147483648 cannot be represented in type 'int32_t' (aka 'int'); cast to an unsigned type to negate this value to itself
Fixes: 2879/clusterfuzz-testcase-minimized-6317542639403008

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

libavcodec/pixlet.c

@@ -331,6 +331,9 @@ static int read_highpass(AVCodecContext *avctx, uint8_t *ptr, int plane, AVFrame
             return AVERROR_INVALIDDATA;
         }
 
+        if (a == INT32_MIN)
+            return AVERROR_INVALIDDATA;
+
         ret = read_high_coeffs(avctx, ptr + bytestream2_tell(&ctx->gb), dest, size,
                                c, (b >= FFABS(a)) ? b : a, d,
                                ctx->band[plane][i + 1].width, stride);