drm/vmwgfx: Fix possible null pointer derefence with invalid contexts

vmw_context_cotable can return either an error or a null pointer and its
usage sometimes went unchecked. Subsequent code would then try to access
either a null pointer or an error value.

The invalid dereferences were only possible with malformed userspace
apps which never properly initialized the rendering contexts.

Check the results of vmw_context_cotable to fix the invalid derefs.

Thanks:
ziming zhang(@ezrak1e) from Ant Group Light-Year Security Lab
who was the first person to discover it.
Niels De Graef who reported it and helped to track down the poc.

Fixes: 9c079b8ce8bf ("drm/vmwgfx: Adapt execbuf to the new validation api")
Cc: <[email protected]> # v4.20+
Reported-by: Niels De Graef  <[email protected]>
Signed-off-by: Zack Rusin <[email protected]>
Cc: Martin Krastev <[email protected]>
Cc: Maaz Mombasawala <[email protected]>
Cc: Ian Forbes <[email protected]>
Cc: Broadcom internal kernel review list <[email protected]>
Cc: [email protected]
Reviewed-by: Maaz Mombasawala <[email protected]>
Reviewed-by: Martin Krastev <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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