wined3d: Avoid some common invalid context accesses.

dlls/wined3d/device.c

@@ -4369,6 +4369,12 @@ HRESULT IWineD3DDeviceImpl_ClearSurface(IWineD3DDeviceImpl *This, IWineD3DSurfac
     }
 
     context = context_acquire(This, (IWineD3DSurface *)target, CTXUSAGE_CLEAR);
+    if (!context->valid)
+    {
+        context_release(context);
+        WARN("Invalid context, skipping clear.\n");
+        return WINED3D_OK;
+    }
 
     target->get_drawable_size(context, &drawable_width, &drawable_height);
 
@@ -5842,6 +5848,13 @@ void stretch_rect_fbo(IWineD3DDevice *iface, IWineD3DSurface *src_surface, WINED
     else if (!surface_is_offscreen(dst_surface)) context = context_acquire(This, dst_surface, CTXUSAGE_RESOURCELOAD);
     else context = context_acquire(This, NULL, CTXUSAGE_RESOURCELOAD);
 
+    if (!context->valid)
+    {
+        context_release(context);
+        WARN("Invalid context, skipping blit.\n");
+        return;
+    }
+
     gl_info = context->gl_info;
 
     if (!surface_is_offscreen(src_surface))

dlls/wined3d/drawprim.c

@@ -596,6 +596,12 @@ void drawPrimitive(IWineD3DDevice *iface, UINT index_count, UINT StartIdx, UINT
     This->isInDraw = TRUE;
 
     context = context_acquire(This, This->render_targets[0], CTXUSAGE_DRAWPRIM);
+    if (!context->valid)
+    {
+        context_release(context);
+        WARN("Invalid context, skipping draw.\n");
+        return;
+    }
 
     if (This->stencilBufferTarget) {
         /* Note that this depends on the context_acquire() call above to set

dlls/wined3d/swapchain.c

@@ -223,6 +223,12 @@ static HRESULT WINAPI IWineD3DSwapChainImpl_Present(IWineD3DSwapChain *iface, CO
     IWineD3DSwapChain_SetDestWindowOverride(iface, hDestWindowOverride);
 
     context = context_acquire(This->device, This->backBuffer[0], CTXUSAGE_RESOURCELOAD);
+    if (!context->valid)
+    {
+        context_release(context);
+        WARN("Invalid context, skipping present.\n");
+        return WINED3D_OK;
+    }
 
     /* Render the cursor onto the back buffer, using our nifty directdraw blitting code :-) */
     if (This->device->bCursorVisible && This->device->cursorTexture)