swscale/swscale: Fix dereference of stride array before null check

Fixes: CID1396263
Fixes: CID1396271

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/swscale.c

@@ -762,10 +762,8 @@ int attribute_align_arg sws_scale(struct SwsContext *c,
     uint8_t *rgb0_tmp = NULL;
     int macro_height = isBayer(c->srcFormat) ? 2 : (1 << c->chrSrcVSubSample);
     // copy strides, so they can safely be modified
-    int srcStride2[4] = { srcStride[0], srcStride[1], srcStride[2],
-                            srcStride[3] };
-    int dstStride2[4] = { dstStride[0], dstStride[1], dstStride[2],
-                            dstStride[3] };
+    int srcStride2[4];
+    int dstStride2[4];
     int srcSliceY_internal = srcSliceY;
 
     if (!srcStride || !dstStride || !dst || !srcSlice) {
@@ -773,6 +771,11 @@ int attribute_align_arg sws_scale(struct SwsContext *c,
         return 0;
     }
 
+    for (i=0; i<4; i++) {
+        srcStride2[i] = srcStride[i];
+        dstStride2[i] = dstStride[i];
+    }
+
     if ((srcSliceY & (macro_height-1)) ||
         ((srcSliceH& (macro_height-1)) && srcSliceY + srcSliceH != c->srcH) ||
         srcSliceY + srcSliceH > c->srcH) {