07-tendermint: safety fixes in Tendermint light client API usage

[cwgoes]

1. cosmos-sdk/x/ibc/light-clients/07-tendermint/client/cli/tx.go

   Line 267 in c8afb2c

   func parseFraction(fraction string) (types.Fraction, error) {

   there's tmfraction.ParseFraction

2. cosmos-sdk/x/ibc/light-clients/07-tendermint/types/misbehaviour.go

   Line 154 in c8afb2c

   voteSet := tmtypes.CommitToVoteSet(chainID, tmCommit, tmValset)

   it might be a better idea to use VerifyCommit or VerifyCommitLight (depending on whenever you need to verify all the signatures or not). CommitToVoteSet panics if it encounters any errors (as stated in the func docs). For example, I don't see where you check that the number of signatures in the commit matches the number of validators. If it's possible to pass such a commit & vals, CommitToVoteSet will panic NOTE: haven't verified it

(thanks to @melekes)