Fixed many crashes in map voting screen related to gametypes with low amount of supported maps.

Mapvote: reduce the number of attempts to add a random map to the vote screen by making them proportional to the number of available maps

Since each attempt to add a random map is costly, it fixes huge lags (9 seconds long in my machine) when the map vote shows up if only a few maps are available for a certain mode, for example with the only official Assault map and the 2 official Onslaught maps. In these case with this patch the shuffle algorithm is called 90 and 99 times less respectively.

It should mitigate or hopefully get rid of #2648, please give feedback.