LRUCache is not exception safe (potential bug)

https://github.com/ClickHouse/ClickHouse/blob/055a6563d756e3535774fdd1cee5b83d7b798801/src/Common/LRUCache.h#L283

If `insert` throws exception (bad_alloc or similar), the Cell will be in incorrect state.

Found by @nvartolomei 