proresdec: Fix read via negative index in a global array.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 0065080320)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/proresdec2.c

@@ -302,7 +302,7 @@ static av_always_inline void decode_dc_coeffs(GetBitContext *gb, DCTELEM *out,
     code = 5;
     sign = 0;
     for (i = 1; i < blocks_per_slice; i++, out += 64) {
-        DECODE_CODEWORD(code, dc_codebook[FFMIN(code, 6)]);
+        DECODE_CODEWORD(code, dc_codebook[FFMIN(code, 6U)]);
         if(code) sign ^= -(code & 1);
         else     sign  = 0;
         prev_dc += (((code + 1) >> 1) ^ sign) - sign;