Overallocate label index buffer

I noticed that you can improve performance on some of the benchmarks by overallocating the index buffer used by `Split` for the average case. Almost 2x faster in some cases, and cuts memory allocations in half for a few.


```
go test -bench . -cpu 1 -benchmem -benchtime 15s github.com/miekg/dns
---
PASS
BenchmarkMsgLen	        10000000	      2523 ns/op	     616 B/op	      14 allocs/op
BenchmarkMsgLenPack	     5000000	      8347 ns/op	    1932 B/op	      56 allocs/op
BenchmarkSplitLabels	100000000	       332 ns/op	      56 B/op	       3 allocs/op
BenchmarkLenLabels	    500000000	      58.7 ns/op	       0 B/op	       0 allocs/op
BenchmarkCompareLabels	50000000	       881 ns/op	     160 B/op	       8 allocs/op
BenchmarkIsSubDomain	10000000	      2468 ns/op	     384 B/op	      22 allocs/op
BenchmarkServing	    1000000	         28474 ns/op	     939 B/op	      26 allocs/op
BenchmarkServingCompress 1000000	     27982 ns/op	     939 B/op	      26 allocs/op
ok  	github.com/miekg/dns	284.668s
```
```
go test -bench . -cpu 1 -benchmem -benchtime 15s github.com/millerkil/dns
---
PASS
BenchmarkMsgLen	        10000000	      2433 ns/op	     592 B/op	      12 allocs/op *
BenchmarkMsgLenPack	     5000000	      8433 ns/op	    1932 B/op	      56 allocs/op
BenchmarkSplitLabels	100000000	       182 ns/op	      32 B/op	       1 allocs/op *
BenchmarkLenLabels	    500000000	      60.6 ns/op	       0 B/op	       0 allocs/op
BenchmarkCompareLabels	50000000	       598 ns/op	     112 B/op	       4 allocs/op *
BenchmarkIsSubDomain	20000000	      1844 ns/op	     304 B/op	      12 allocs/op *
BenchmarkServing	    1000000	         28659 ns/op	     934 B/op	      26 allocs/op
BenchmarkServingCompress 1000000	     28552 ns/op	     936 B/op	      26 allocs/op
ok  	github.com/millerkil/dns	267.728s
```