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pack: make LRU cache work when fully drained.

The added checks are only needed when the LRU
cache is fully drained, which should never be
the case: it should grow to the expected size,
and then stay roughly steady.

However, a reader would need to convince
themselves of this when inspecting the code,
so this removes a few questions.

--- a/pack.c

+++ b/pack.c

@@ -140,7 +140,8 @@

 		o->next->prev = o->prev;

 	if(lrutail == o){

 		lrutail = o->prev;

-		lrutail->next = nil;

+		if(lrutail != nil)

+			lrutail->next = nil;

 	}else if(lrutail == nil)

 		lrutail = o;

 	if(lruhead)

@@ -154,10 +155,11 @@

 		ref(o);

 		ncache++;

 	}

-	while(ncache > cachemax){

+	while(ncache > cachemax && lrutail != nil){

 		p = lrutail;

 		lrutail = p->prev;

-		lrutail->next = nil;

+		if(lrutail != nil)

+			lrutail->next = nil;

 		p->flag &= ~Ccache;

 		p->prev = nil;

 		p->next = nil;