Fix segfault when spectating on an Emerald Hunt map

If you create a custom gametype that allows spectating on Emerald Hunt maps, spectators can trigger segfaults by simply joining the game. This happens because spectators have one single tic where player->mo is NULL while joining, which is enough to trigger a NULL-dereference in the Emerald Hunt HUD logic used to detect the distance from an emerald.

This SOC can be used to trigger the bug:

GameType Spectator test
TypeOfLevel = Coop
Identifier = "spectest"
Rules = GTR_CAMPAIGN|GTR_LIVES|GTR_FRIENDLY|GTR_SPAWNENEMIES|GTR_ALLOWEXIT|GTR_EMERALDHUNT|GTR_EMERALDTOKENS|GTR_SPECIALSTAGES|GTR_CUTSCENES|GTR_SPECTATORS|GTR_NOSPECTATORSPAWN
HeaderColor = 133
RankingType = GT_COOP
IntermissionType = int_coop
Description = ""
#

To reproduce:

• Load the SOC above with a map pack that contains an Emerald Hunt map (CyberDime Realm and Sol Sestancia are two I know of).
• Start a server with the custom gametype on any Emerald Hunt map.
• Go into spectator mode.
• Join back into the game.

src/st_stuff.c

@@ -2512,6 +2512,8 @@ num:
 static INT32 ST_drawEmeraldHuntIcon(mobj_t *hunt, patch_t **patches, INT32 offset)
 {
 	INT32 interval, i;
+	if (stplyr->mo == NULL)
+		return 0;  // player just joined after spectating, can happen on custom gamemodes.
 	UINT32 dist = ((UINT32)P_AproxDistance(P_AproxDistance(stplyr->mo->x - hunt->x, stplyr->mo->y - hunt->y), stplyr->mo->z - hunt->z))>>FRACBITS;
 
 	if (dist < 128)