Handle evictions properly when cache is empty.

go.mod

@@ -2,4 +2,4 @@ module git.wntrmute.dev/kyle/go-mru
 
 go 1.17
 
-require github.com/benbjohnson/clock v1.3.0
+require github.com/benbjohnson/clock v1.3.5

go.sum

@@ -0,0 +1,2 @@
+github.com/benbjohnson/clock v1.3.5 h1:VvXlSJBzZpA/zum6Sj74hxwYI2DIxRWuNIoXAzHZz5o=
+github.com/benbjohnson/clock v1.3.5/go.mod h1:J11/hYXuz8f4ySSvYwY0FKfm+ezbsZBKZxNJlLklBHA=

mru.go

@@ -53,6 +53,10 @@ func (c *Cache) Len() int {
 
 // evict should remove the least-recently-used cache item.
 func (c *Cache) evict() {
+	if c.access.Len() == 0 {
+		return
+	}
+
 	k := c.access.K(0)
 	c.evictKey(k)
 }

mru_test.go

@@ -20,6 +20,11 @@ func TestBasicCacheEviction(t *testing.T) {
 		t.Fatal("cache should have size 0")
 	}
 
+	c.evict()
+	if err := c.ConsistencyCheck(); err != nil {
+		t.Fatal(err)
+	}
+
 	c.Store("raven", 1)
 	if err := c.ConsistencyCheck(); err != nil {
 		t.Fatal(err)