common: NewBitArray never crashes on negatives (#170)

Fixes #169
Fixes https://github.com/tendermint/tendermint/issues/1322

The previous code was very trusting assuming that
rational actors will use this code. However, Byzantine
actors don't care and in the case of the linked issue
negative lengths can be sent to this code unfettered
having been received from a peer.

This code is essentially just a sign change from
`==`
to
`<=`

and we've gutted out that attack by being more defensive.

common/bit_array.go

@@ -15,7 +15,7 @@ type BitArray struct {
 
 // There is no BitArray whose Size is 0.  Use nil instead.
 func NewBitArray(bits int) *BitArray {
-	if bits == 0 {
+	if bits <= 0 {
 		return nil
 	}
 	return &BitArray{