Fix potential nulllpointer dereferences

src/g_func.c

@@ -937,6 +937,9 @@ void door_use (edict_t *self, edict_t *other, edict_t *activator)
 {
 	edict_t	*ent;
 
+	if (!self)
+		return;
+
 	if (self->flags & FL_TEAMSLAVE)
 		return;
 

src/g_items.c

@@ -109,6 +109,9 @@ gitem_t	*FindItem (char *pickup_name)
 
 void DoRespawn (edict_t *ent)
 {
+	if (!ent)
+		return;
+
 	if (ent->team)
 	{
 		edict_t	*master;

src/g_weapon.c

@@ -687,6 +687,9 @@ void fire_rail (edict_t *self, vec3_t start, vec3_t aimdir, int damage, int kick
 	int		mask;
 	qboolean	water;
 
+	if (!self)
+		return;
+
 	VectorMA (start, 8192, aimdir, end);
 	VectorCopy (start, from);
 	ignore = self;