drm/rect: Avoid division by zero

Check for zero width/height destination rectangle in
drm_rect_clip_scaled() to avoid a division by zero.

Cc: stable@vger.kernel.org
Fixes: f96bdf56 ("drm/rect: Handle rounding errors in drm_rect_clip_scaled, v3.")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Benjamin Gaignard <benjamin.gaignard@st.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Testcase: igt/kms_selftest/drm_rect_clip_scaled_div_by_zero
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191122175623.13565-2-ville.syrjala@linux.intel.comReviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@st.com>

drivers/gpu/drm/drm_rect.c

@@ -54,7 +54,12 @@ EXPORT_SYMBOL(drm_rect_intersect);
 
 static u32 clip_scaled(u32 src, u32 dst, u32 clip)
 {
-	u64 tmp = mul_u32_u32(src, dst - clip);
+	u64 tmp;
+
+	if (dst == 0)
+		return 0;
+
+	tmp = mul_u32_u32(src, dst - clip);
 
 	/*
 	 * Round toward 1.0 when clipping so that we don't accidentally