mesa: Fix glGetActiveAttribute for gl_VertexID when lowered.

mesa: Fix glGetActiveAttribute for gl_VertexID when lowered.

The lower_vertex_id pass converts uses of the gl_VertexID system value
to the gl_BaseVertex and gl_VertexIDMESA system values.  Since
gl_VertexID is no longer accessed, it would not be considered active.

Of course, it should be, since the shader uses gl_VertexID.

v2: Move the var->name dereference past the var != NULL check.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>