common: fix BitArray.Update to avoid nil dereference

Update previously only checked that the receiver was
non-nil but didn't check that the input parameter to update
"o" was non-nil causing a nil dereference in cases such as
fe632ea32a/consensus/reactor.go (L306)

Fixes https://github.com/tendermint/tendermint/issues/1169

common/bit_array.go

@@ -306,7 +306,7 @@ func (bA *BitArray) Bytes() []byte {
 // so if necessary, caller must copy or lock o prior to calling Update.
 // If bA is nil, does nothing.
 func (bA *BitArray) Update(o *BitArray) {
-	if bA == nil {
+	if bA == nil || o == nil {
 		return
 	}
 	bA.mtx.Lock()