mesa: Throw an error when starting conditional render on an active query.

mesa: Throw an error when starting conditional render on an active query.

From the NV_conditional_render spec:

    BeginQuery sets the active query object name for the query type given by
    <target> to <id>.  If BeginQuery is called with an <id> of zero, if the
    active query object name for <target> is non-zero, if <id> is the active
    query object name for any query type, or if <id> is the active query
    object for condtional rendering (Section 2.X), the error INVALID OPERATION
    is generated.

Fixes piglit nv_conditional_render-begin-while-active.

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