gpu: drm: radeon: Fix a possible null-pointer dereference in radeon_connector_set_property()

In radeon_connector_set_property(), there is an if statement on line 743
to check whether connector->encoder is NULL:
    if (connector->encoder)

When connector->encoder is NULL, it is used on line 755:
    if (connector->encoder->crtc)

Thus, a possible null-pointer dereference may occur.

To fix this bug, connector->encoder is checked before being used.

This bug is found by a static analysis tool STCheck written by us.
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/radeon_connectors.c

@@ -752,7 +752,7 @@ static int radeon_connector_set_property(struct drm_connector *connector, struct
 
 		radeon_encoder->output_csc = val;
 
-		if (connector->encoder->crtc) {
+		if (connector->encoder && connector->encoder->crtc) {
 			struct drm_crtc *crtc  = connector->encoder->crtc;
 			struct radeon_crtc *radeon_crtc = to_radeon_crtc(crtc);