lavc/proresdec2: Do not mix variable declaration and statement.

Fixes ticket #6728.

libavcodec/proresdec2.c

@@ -598,8 +598,9 @@ static int decode_slice_thread(AVCodecContext *avctx, void *arg, int jobnr, int
     }
     else {
         size_t mb_max_x = slice->mb_count << (mb_x_shift - 1);
-        for (size_t i = 0; i < 16; ++i)
-            for (size_t j = 0; j < mb_max_x; ++j) {
+        size_t i, j;
+        for (i = 0; i < 16; ++i)
+            for (j = 0; j < mb_max_x; ++j) {
                 *(uint16_t*)(dest_u + (i * chroma_stride) + (j << 1)) = 511;
                 *(uint16_t*)(dest_v + (i * chroma_stride) + (j << 1)) = 511;
             }