avcodec/mss12: Don't apply non-zero offset to null pointer

Affected the FATE tests mss2-wmv and mss1-pal.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

libavcodec/mss12.c

@@ -291,14 +291,15 @@ static int decode_pixel_in_context(ArithCoder *acoder, PixContext *pctx,
         return decode_pixel(acoder, pctx, ref_pix, nlen, 1);
 }
 
-static int decode_region(ArithCoder *acoder, uint8_t *dst, uint8_t *rgb_pic,
+static int decode_region(ArithCoder *acoder, uint8_t *dst, uint8_t *rgb_dst,
                          int x, int y, int width, int height, ptrdiff_t stride,
                          ptrdiff_t rgb_stride, PixContext *pctx,
                          const uint32_t *pal)
 {
     int i, j, p;
-    uint8_t *rgb_dst = rgb_pic + x * 3 + y * rgb_stride;
 
+    rgb_stride = rgb_dst ? rgb_stride : 0;
+    rgb_dst    = rgb_dst ? rgb_dst + x * 3 + y * rgb_stride : NULL;
     dst += x + y * stride;
 
     for (j = 0; j < height; j++) {
@@ -312,11 +313,11 @@ static int decode_region(ArithCoder *acoder, uint8_t *dst, uint8_t *rgb_pic,
                 return p;
             dst[i] = p;
 
-            if (rgb_pic)
+            if (rgb_dst)
                 AV_WB24(rgb_dst + i * 3, pal[p]);
         }
         dst     += stride;
-        rgb_dst += rgb_stride;
+        rgb_dst  = FF_PTR_ADD(rgb_dst, rgb_stride);
     }
 
     return 0;
@@ -476,17 +477,19 @@ static int decode_region_intra(SliceContext *sc, ArithCoder *acoder,
         ptrdiff_t stride     = c->pal_stride;
         ptrdiff_t rgb_stride = c->rgb_stride;
         uint8_t *dst     = c->pal_pic + x     + y * stride;
-        uint8_t *rgb_dst = c->rgb_pic + x * 3 + y * rgb_stride;
+        uint8_t *rgb_dst = c->rgb_pic ? c->rgb_pic + x * 3 + y * rgb_stride : NULL;
 
         pix     = decode_pixel(acoder, &sc->intra_pix_ctx, NULL, 0, 0);
         if (pix < 0)
             return pix;
         rgb_pix = c->pal[pix];
-        for (i = 0; i < height; i++, dst += stride, rgb_dst += rgb_stride) {
+        for (i = 0; i < height; i++, dst += stride) {
             memset(dst, pix, width);
-            if (c->rgb_pic)
+            if (rgb_dst) {
                 for (j = 0; j < width * 3; j += 3)
                     AV_WB24(rgb_dst + j, rgb_pix);
+                rgb_dst += rgb_stride;
+            }
         }
     } else {
         return decode_region(acoder, c->pal_pic, c->rgb_pic,