avcodec/hdrdec: Update w in inner loop of decompress()

Fixes: out of array access
Fixes: 50936/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HDR_fuzzer-5423041009549312

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

libavcodec/hdrdec.c

@@ -70,8 +70,8 @@ static int decompress(uint8_t *scanline, int w, GetByteContext *gb, const uint8_
             for (int i = run << rshift; i > 0 && w > 0 && scanline >= start + 4; i--) {
                 memcpy(scanline, scanline - 4, 4);
                 scanline += 4;
+                w -= 4;
             }
-            w -= run << rshift;
             rshift += 8;
             if (rshift > 16)
                 break;