solve panic.

[liyan-ah]

solve panic in:
#145

The same issue happened to me. I add RWMutex outside and add RLock before get and Lock before Add. But outside lock seems not work, I got the same panic.
It seems that a type check is needed.
memory leak would be involved since delete(c.cache, kv.key) maybe ignored.

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[liyan-ah]

cc @neild @bradfitz @desimone

[liyan-ah]

any one check this?
cc @neild @bradfitz @desimone

[bcmills]

This looks like the wrong place for a fix. (Why is removeElement being called on an element with an invalid entry in the first place?)

[liyan-ah]

any ideas?
my code just like this:

func A(){
    GlobalMutex.Lock()
    lru.Add(key, value)
    GlobalMutex.Unlock()
}

func B(){
   GlobalMutex.RLock()
   lru.Get(key)
   GlobalMutex.RUnlock()
}
// A and B may be called in multi-goroutines.

only Add() and Get() be called. From the source code, I didn't find out any problem. But it do paniced.