swscale: prevent invalid writes in packed_16bpc_bswap

Writes past the end of the destination buffer were occuring when its
stride was smaller than the stride of the source. Fixes Bug #183.

libswscale/swscale_unscaled.c

@@ -285,9 +285,10 @@ static int packed_16bpc_bswap(SwsContext *c, const uint8_t *src[],
     int dststr = dstStride[0] >> 1;
     uint16_t       *dstPtr =       (uint16_t *) dst[0];
     const uint16_t *srcPtr = (const uint16_t *) src[0];
+    int min_stride         = FFMIN(srcstr, dststr);
 
     for (i = 0; i < srcSliceH; i++) {
-        for (j = 0; j < srcstr; j++) {
+        for (j = 0; j < min_stride; j++) {
             dstPtr[j] = av_bswap16(srcPtr[j]);
         }
         srcPtr += srcstr;