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_edid.c
The call to drm_cvt_mode() in function drm_mode_std() for the
HDTV hack resulted in the possibility of accessing a NULL pointer
if drm_mode_std() returned NULL.  A check for this added right after
the call to drm_cvt_mode() in this particular area of code.

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/20180212195144.98323-4-joe.moriarty@oracle.com