Convert index checks to assert, giving a significant speed boost.

Before:
```
Benchmark                                        Mode Thr    Cnt  Sec         Mean   Mean error    Units
n.r.b.FloatBufferBenchmarks.testBufferAccess    thrpt   1      4    1       97.465        0.234   ops/ms
n.r.b.FloatBufferBenchmarks.testDirectAccess    thrpt   1      4    1      100.365        2.625   ops/ms
n.r.b.FloatBufferBenchmarks.testHeapAccess      thrpt   1      4    1       98.636        2.494   ops/ms
n.r.b.FloatBufferBenchmarks.testSliceAccess     thrpt   1      4    1       63.436        5.909   ops/ms
```

After, with `-ea`:
```
Benchmark                                        Mode Thr    Cnt  Sec         Mean   Mean error    Units
n.r.b.FloatBufferBenchmarks.testBufferAccess    thrpt   1      4    1       97.700        1.689   ops/ms
n.r.b.FloatBufferBenchmarks.testDirectAccess    thrpt   1      4    1       94.973        7.757   ops/ms
n.r.b.FloatBufferBenchmarks.testHeapAccess      thrpt   1      4    1       98.593        5.100   ops/ms
n.r.b.FloatBufferBenchmarks.testSliceAccess     thrpt   1      4    1       71.493        0.777   ops/ms
```

After, with `-da`:
```
Benchmark                                        Mode Thr    Cnt  Sec         Mean   Mean error    Units
n.r.b.FloatBufferBenchmarks.testBufferAccess    thrpt   1      4    1      100.675        2.361   ops/ms
n.r.b.FloatBufferBenchmarks.testDirectAccess    thrpt   1      4    1       99.485        1.123   ops/ms
n.r.b.FloatBufferBenchmarks.testHeapAccess      thrpt   1      4    1      101.129        1.471   ops/ms
n.r.b.FloatBufferBenchmarks.testSliceAccess     thrpt   1      4    1       96.282        2.697   ops/ms
```

Benchmark code: https://gist.github.com/stevenschlansker/7143614