Bug in previousClearBit

Hi,

while testing the 1.2 version I found the following issue:
```
SparseBitSet set = new SparseBitSet();
set.set(0);
set.set(64);
System.out.println(set.previousClearBit(64));
set.clear(0);
set.set(1);
System.out.println(set.previousClearBit(64));
```

Expected: (consistent with BitSet)
```
63
63
```
Got:
```
-1
0
```

@BrentDouglas 