Don't call the driver clear hook when the effective scissor rectangle is empty.

src/mesa/main/buffers.c

@@ -140,7 +140,9 @@ _mesa_Clear( GLbitfield mask )
       return;
    }
 
-   if (ctx->DrawBuffer->Width == 0 || ctx->DrawBuffer->Height == 0)
+   if (ctx->DrawBuffer->Width == 0 || ctx->DrawBuffer->Height == 0 ||
+       ctx->DrawBuffer->_Xmin >= ctx->DrawBuffer->_Xmax ||
+       ctx->DrawBuffer->_Ymin >= ctx->DrawBuffer->_Ymax)
       return;
 
    if (ctx->RenderMode == GL_RENDER) {