gallium/u_vbuf: Protect against overflow with large instance divisors.

gallium/u_vbuf: Protect against overflow with large instance divisors.

GTF-GLES3.gtf.GL3Tests.instanced_arrays.instanced_arrays_divisor uses -1
as a divisor, so we would overflow to count=0 and upload no data,
triggering the assert below.  We want to upload 1 element in this case,
fixing the test on VC5.

v2: Use some more obvious logic, and explain why we don't use the normal
    round_up().

Reviewed-by: Brian Paul <brianp@vmware.com>