Fix error where config.Shards = 0 causes "index out of range"

[KarlAustin]

Issue Fixed:

If config.Shards is set to 0 then an "index out of range" error will be generated by bigcache.GetShard() at bigcache.go:220

runtime error: index out of range [6004679098338124500] with length 0
goroutine 98 [running]:
net/http.(*conn).serve.func1(0xc00016c000)
/usr/lib/go-1.13/src/net/http/server.go:1767 +0x139
panic(0xa5e780, 0xc000166160)
/usr/lib/go-1.13/src/runtime/panic.go:679 +0x1b2
github.com/allegro/bigcache.(*BigCache).getShard(...)
...github.com/allegro/bigcache/bigcache.go:220

func (c *BigCache) getShard(hashedKey uint64) (shard *cacheShard) { return c.shards[hashedKey&c.shardMask] }

As the isPowerofTwo() check considers 0 a power of two.

Test:

TestClosing (bicache_test.go) failed because it didn't specify a number of Shards in its config.

All tests pass with the minor change to TestClosing.

[cristaloleg]

Thanks!

[cristaloleg]

Looks like go fmt should be run https://github.com/allegro/bigcache/pull/280/checks?check_run_id=2773331575#step:4:23

[codecov-commenter]

Codecov Report

Merging #280 (966cbfd) into master (eb759b5) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #280   +/-   ##
=======================================
  Coverage   86.52%   86.52%           
=======================================
  Files          15       15           
  Lines         638      638           
=======================================
  Hits          552      552           
  Misses         71       71           
  Partials       15       15           

Impacted Files	Coverage Δ
utils.go	62.50% <100.00%> (ø)

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