check for isset in nodeapi update

when saving a node in unpublished state we're encounting several errors.. see

warning: array_intersect() [function.array-intersect]: Argument #2 is not an array in /home/web/sites/all/modules/og_statistics/og_statistics.module on line 102.
warning: array_diff() [function.array-diff]: Argument #2 is not an array in /home/web/sites/all/modules/og_statistics/og_statistics.module on line 103.
warning: array_diff() [function.array-diff]: Argument #1 is not an array in /home/web/sites/all/modules/og_statistics/og_statistics.module on line 104.
warning: Invalid argument supplied for foreach() in /home/web/sites/all/modules/og_statistics/og_statistics.module on line 105.
warning: Invalid argument supplied for foreach() in /home/web/sites/all/modules/og_statistics/og_statistics.module on line 108.
warning: Invalid argument supplied for foreach() in /home/web/sites/all/modules/og_statistics/og_statistics.module on line 112.

Seems like $node->og_groups is not set in case of unpublished nodes - even if $node->og_groups is present.

Attached patch checks for isset($node->og_groups) leading to a correct issuesless result.