Panic when querying for non existing value with pilosa index

```
CREATE INDEX email_idx ON commits USING pilosa (commit_author_email);
SELECT count(*) FROM commits WHERE  commit_author_email='existing email' OR commit_author_email='trash';
```

Got:
```
...
lookup.go:87 (0x4927553)
(*indexLookup).bitmapQuery: if b.Error() != nil {
```

Fix:
```
b, err := il.bitmapQuery()
if err != nil {
return nil, err
}
 if b == nil {
continue
}
```