wined3d: Return bools from all return paths in wined3d_cs_map_upload_bo().

dlls/wined3d/cs.c

@@ -2968,13 +2968,13 @@ static bool wined3d_cs_map_upload_bo(struct wined3d_device_context *context, str
         if (!d3d_info->xyzrhw || !d3d_info->vertex_bgra || !d3d_info->ffp_generic_attributes)
         {
             TRACE("Not returning a persistent buffer because we might need to do vertex attribute conversion.\n");
-            return NULL;
+            return false;
         }
 
         if (resource->pin_sysmem)
         {
             TRACE("Not allocating an upload buffer because system memory is pinned for this resource.\n");
-            return NULL;
+            return false;
         }
 
         if ((flags & WINED3D_MAP_NOOVERWRITE) && client->addr.buffer_object == CLIENT_BO_DISCARDED)