fix(buffer): make buffer capacity atleast defaultCapacity

[NamanJain8]

z.Buffer when initialized with sz < 8, it may cause crash as the buffer size is less than offset.

panic: runtime error: slice bounds out of range [:8] with capacity 7 [recovered]
	panic: runtime error: slice bounds out of range [:8] with capacity 7

goroutine 6 [running]:
testing.tRunner.func1.2(0xa09780, 0xc00084e090)
	/usr/local/go/src/testing/testing.go:1143 +0x332
testing.tRunner.func1(0xc000001b00)
	/usr/local/go/src/testing/testing.go:1146 +0x4b6
panic(0xa09780, 0xc00084e090)
	/usr/local/go/src/runtime/panic.go:965 +0x1b9
github.com/dgraph-io/ristretto/z.(*Buffer).Grow(0xc0067f60e0, 0xf)
	/home/algod/go/src/github.com/dgraph-io/ristretto/z/buffer.go:230 +0x7c5
github.com/dgraph-io/ristretto/z.(*Buffer).Write(...)
	/home/algod/go/src/github.com/dgraph-io/ristretto/z/buffer.go:495
github.com/dgraph-io/ristretto/z.(*sortHelper).sortSmall(0xc000135840, 0x8, 0x17)
	/home/algod/go/src/github.com/dgraph-io/ristretto/z/buffer.go:328 +0x305
github.com/dgraph-io/ristretto/z.(*Buffer).SortSliceBetween(0xc0001fe070, 0x8, 0x17, 0xa61b90)
	/home/algod/go/src/github.com/dgraph-io/ristretto/z/buffer.go:448 +0x453
github.com/dgraph-io/ristretto/z.(*Buffer).SortSlice(...)
	/home/algod/go/src/github.com/dgraph-io/ristretto/z/buffer.go:411
github.com/dgraph-io/badger/v3.(*DB).DropPrefixNonBlocking.func1(0xc000156001, 0xae8df0, 0xc00001c140)
	/home/algod/go/src/github.com/dgraph-io/badger/db.go:1866 +0x155
github.com/dgraph-io/badger/v3.(*DB).DropPrefixNonBlocking.func2(0xc0008561f8, 0x2, 0x2, 0xc0001fe070, 0x1)
	/home/algod/go/src/github.com/dgraph-io/badger/db.go:1935 +0x21f
github.com/dgraph-io/badger/v3.(*DB).DropPrefixNonBlocking(0xc000123b00, 0xc000850a80, 0x1, 0x1, 0x0, 0x0)
	/home/algod/go/src/github.com/dgraph-io/badger/db.go:1940 +0x24c
github.com/dgraph-io/badger/v3.TestDropPrefixNonBlockingNoError(0xc000001b00)
	/home/algod/go/src/github.com/dgraph-io/badger/db2_test.go:1150 +0x49c
testing.tRunner(0xc000001b00, 0xa62018)
	/usr/local/go/src/testing/testing.go:1193 +0xef
created by testing.(*T).Run
	/usr/local/go/src/testing/testing.go:1238 +0x2b3

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This change is 

[manishrjain]

Reviewed 2 of 2 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @jarifibrahim)