i965: Assert that the offset in the VBO is below the VBO size.

i965: Assert that the offset in the VBO is below the VBO size.

This avoids sending a bad buffer address to the GPU due to programmer error,
and is permitted by the ARB_vbo spec.  Note that we still have the opportunity
to dereference past the end of the GPU, because we aren't clipping to a
correct _MaxElement, but that appears to be harder than it should be.  This
gets us the 90% solution.

Bug #19911.