drm: NULL pointer dereference [null-pointer-deref] (CWE 476) problem

The Parfait (version 2.1.0) static code analysis tool found the
following NULL pointer derefernce problem.

- drivers/gpu/drm/drm_vblank.c
Null pointer checks were added to return values from calls to
drm_crtc_from_index().  There is a possibility, however minute, that
crtc->index may not be found when trying to find the struct crtc
from it's assigned index given in drm_crtc_init_with_planes().
3 return checks for NULL where added with a call to
WARN_ON(!crtc).

Signed-off-by: Joe Moriarty <joe.moriarty@oracle.com>
Reviewed-by: Steven Sistare <steven.sistare@oracle.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180220191157.100960-2-joe.moriarty@oracle.com

drivers/gpu/drm/drm_vblank.c

@@ -120,6 +120,9 @@ static u32 __get_vblank_counter(struct drm_device *dev, unsigned int pipe)
 	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
 		struct drm_crtc *crtc = drm_crtc_from_index(dev, pipe);
 
+		if (WARN_ON(!crtc))
+			return 0;
+
 		if (crtc->funcs->get_vblank_counter)
 			return crtc->funcs->get_vblank_counter(crtc);
 	}
@@ -318,6 +321,9 @@ static void __disable_vblank(struct drm_device *dev, unsigned int pipe)
 	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
 		struct drm_crtc *crtc = drm_crtc_from_index(dev, pipe);
 
+		if (WARN_ON(!crtc))
+			return;
+
 		if (crtc->funcs->disable_vblank) {
 			crtc->funcs->disable_vblank(crtc);
 			return;
@@ -918,6 +924,9 @@ static int __enable_vblank(struct drm_device *dev, unsigned int pipe)
 	if (drm_core_check_feature(dev, DRIVER_MODESET)) {
 		struct drm_crtc *crtc = drm_crtc_from_index(dev, pipe);
 
+		if (WARN_ON(!crtc))
+			return 0;
+
 		if (crtc->funcs->enable_vblank)
 			return crtc->funcs->enable_vblank(crtc);
 	}