Tendermint version (use tendermint version or git rev-parse --verify HEAD if installed from source):
Version 170cb70

What happened:
Auditing dependencies for the cosmos-sdk and I noticed this code

Problems in it:

a) If protoBitArray is nil, it tries to set the pointer receiver to nil; but that operation doesn't reflect on the outside, because of the way pointer receivers are created demoed at https://play.golang.org/p/6IXb-hsVJ7s or inlined below

package main

import "fmt"

func main() {
	s := &sk{dt: []int{12, 19}}
	s.fromOther(nil)
	fmt.Printf("%#v\n", s)
}

type sk struct {
	dt []int
}

func (s *sk) fromOther(dk *sk) {
	if dk == nil {
		s = nil
		return
	}
	s.dt = dk.dt
}

it'll still print the prior result

&main.sk{dt:[]int{12, 19}}

Did we mean to instead clear out all the fields of the receiver?
It doesn't work because to change a pointer value, you need an lvalue aka a variable receiving it. If it were a **, then one could do *s = nil.

b) FromBitArray blindly reuses the slice passed in from its arguments; which means that someone can later change the underlying slice and mutate the end result
c) ToProto also blindly passes in the underlying slice

For both b) and c) we should make a copy of the slice Elems.