swrast: Fix signed/unsigned problems with negative strides.

In swrast_map_renderbuffer negative strides lead to
render buffer map pointers that are off by 2^32.
Make sure that intermediate negative values are not
converted to an unsigned.

Signed-off-by: Mathias Froehlich <Mathias.Froehlich@web.de>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/drivers/dri/swrast/swrast.c

@@ -419,8 +419,8 @@ swrast_map_renderbuffer(struct gl_context *ctx,
       stride = -stride;
    }
 
-   map += y * stride;
-   map += x * cpp;
+   map += (GLsizei)y * stride;
+   map += (GLsizei)x * cpp;
 
    *out_map = map;
    *out_stride = stride;