swr: fix index buffers with non-zero indices

swr: fix index buffers with non-zero indices

Fix issue with index buffers that do not contain a 0 index.  0 index
can be a non-valid index if the (copied) vertex buffers are a subset of the
user's (which happens because we only copy the range between min & max).
Core will use an index passed in from the driver to replace invalid indices.

Only do this for calls that contain non-zero indices, to minimize performance

Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>
cost.