Check if current server entry is not null

[Jerozgen]

This should prevent crashes while using Replay Mod:

java.lang.NullPointerException: Cannot read field "field_3761" because the return value of "net.minecraft.class_310.method_1558()" is null
	at com.holebois.client_maps.client_mapsClient.getMap(client_mapsClient.java:34)
	at net.minecraft.class_1806.handler$bjc000$client_maps$getMapState(class_1806.java:539)
	at net.minecraft.class_1806.method_7997(class_1806.java:60)
	...

I didn't test this though