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>

src/mesa/main/condrender.c

@@ -73,7 +73,7 @@ _mesa_BeginConditionalRender(GLuint queryId, GLenum mode)
    }
    ASSERT(q->Id == queryId);
 
-   if (q->Target != GL_SAMPLES_PASSED) {
+   if (q->Target != GL_SAMPLES_PASSED || q->Active) {
       _mesa_error(ctx, GL_INVALID_OPERATION, "glBeginConditionalRender()");
       return;
    }