Fix: intro dropship crash

[Bobbyperson]

This PR fixes an inconsistent crash during the intro dropships, which was introduced in #809. Oddly, this issue only seems to happen on dedicated servers.

[00:47:00] [SCRIPT SV] [info] SCRIPT ERROR: [SERVER] getrandom(): Array is empty
[00:47:00] [SCRIPT SV] [info]  -> playerDropship = introDropships.getrandom()
[00:47:00] [SCRIPT SV] [info]
CALLSTACK
*FUNCTION [PutPlayerInDropship()] mp/_classic_mp_dropship_intro.gnut line [168]
*FUNCTION [DropshipIntro_OnClientConnected()] mp/_classic_mp_dropship_intro.gnut line [50]
*FUNCTION [CodeCallback_OnClientConnectionCompleted()] mp/_base_gametype_mp.gnut line [173]

Why this happens:

When the code runs GetEntArrayByClass_Expensive( "info_spawnpoint_dropship_start" ) to get places to spawn the dropships, sometimes it will run before the server has finished loading all the ents, which can cause this to return an empty or incomplete list. There are no checks for this, so the code will continue running until it tries to put players in the dropship, which will cause it to crash due to the list being empty.

The fix:

The prematch code will now wait until all entities have been loaded (AddCallback_EntitiesDidLoad). Additionally, the loop for spawning dropships has been updated to fix a rare case where too few or no dropships can spawn.

Testing:

This fix (and several other attempts) has been extensively tested on the previous ikhadhonger servers. But to test this, simply start a dedicated server and switch maps several times. Without this PR it will eventually crash.