v3d: Early return with handle 0 when getting a bo on the simulator

v3d: Early return with handle 0 when getting a bo on the simulator

Until now we were just asking entries on the bo hash table, and don't
worry if the handle was NULL, as we were just expecting to get a NULL
in return. It seems that now the hash table assert with some reserverd
pointers, included NULL. This commit just early returns with handle 0.

This change fixes several crashes on vk-gl-cts GLES tests when using
the v3d simulator, like:
KHR-GLES3.core.internalformat.copy_tex_image.*

Reviewed-by: Eric Anholt <eric@anholt.net>