fix: guard LRUCache against non-positive capacity

[nikolauspschuetz]

What

NewLRUCache and NewSyncLRUCache are exported public API, but calling Put on a cache created with a non-positive capacity panics with a nil pointer dereference.

Reproduce:

c := util.NewLRUCache(0)
c.Put("a", 1) // panic: runtime error: invalid memory address or nil pointer dereference

On the first insert, the eviction branch is taken (len(cache.m) >= cache.capacity is 0 >= 0), which calls cache.remove(cache.tail.prev, false). On an empty list cache.tail.prev is the head sentinel, whose prev is nil, so remove dereferences nil at util/util.go.

Fix

Return early from Put when capacity <= 0 — such a cache holds nothing, so there is nothing to store and no eviction to perform. This keeps Get returning (nil, false) for every key, consistent with a zero-capacity cache.

Guarding the eviction path instead would let a zero-capacity cache incorrectly hold one entry, so the early return is the more correct minimal fix.

Test

Added TestLRUCacheNonPositiveCapacity in util/util_test.go. It panics on master and passes with this change:

• before: panic: runtime error: invalid memory address or nil pointer dereference (in Put)
• after: ok github.com/casbin/casbin/v3/util

---

for the record: fix found and drafted with AI assistance; reviewed and verified by me.