As you state depression as a result of CVA probably is not probable mechanism to enhance survival of an organism. However depression itself with flattening of affect, lessening of activity and general energy conserving that would ensue may have aided survival in hostile northern hemisphere winters when food sources are low, and mobility limited. It may also help social creatures who are confined in small spaces to have less social intercourse which could be costly in terms of energy expended and possible conflicts. It could of course be argued the other way around that depression triggered by the onset of colder weather was a quite reasonable response to the prospect of the next few months shut in with the relatives, with not much food to go around.