Fixed a uniform buffer regression

Fixed a uniform buffer regression

The following cl had broken all fragment uniform buffers:
https://swiftshader-review.googlesource.com/6950

Since the null pointer check was only made on the vertex
uniform buffer, it was preventing the fragment uniform
buffer from being set, which was causing a crash when this
happened.

There was a second issue  with this cl, which was allowing
'uniformBuffers[-1].get()' to be called, which should not
have been allowed.

Also, it is safer to call VertexProcessor::setUniformBuffer
and PixelProcessor::setUniformBuffer unconditionally in order
to set the null pointers correctly when no buffers are set.

Change-Id: I275b0bca6173ee3df4e0e369926ed0a94500e1ba
Reviewed-on: https://swiftshader-review.googlesource.com/7216
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>