wire: Correct fuzz test for MsgCommitSig.

[davecgh]

This corrects the fuzz test in TestLightningWireProtocol for MsgCommitSig to avoid creating an empty slice since the decoded message only creates a slice when there are greater than zero signatures and an empty slice is not considered equal to a nil slice under reflection.

This can be tested by running the TestLightningWireProtocol 100 times in a loop with and without this change.

For example, here is the redacted output prior to this PR:

$ for i in $(seq 0 1 100); do go test -run TestLightningWireProtocol; done

...
        lnwire_test.go:127: messages don't match after re-encoding: (*lnwire.CommitSig)(0xc04204a900)({
                 ChanID: (lnwire.ChannelID) (len=32 cap=32) 19fda145e1053c5d36890e5169101beeb80875fea036c0b8de1c7bc66c427739,
                 CommitSig: (*btcec.Signature)(0x8bcac0)({
                  R: (*big.Int)(0xc042003260)(63724406601629180062774974542967536251589935445068131219452686511677818569431),
                  S: (*big.Int)(0xc042003280)(18801056069249825825291287104931333862866033135609736119018462340006816851118)
                 }),
                 HtlcSigs: ([]*btcec.Signature) {
                 }
                })
                 vs (*lnwire.CommitSig)(0xc04204a940)({
                 ChanID: (lnwire.ChannelID) (len=32 cap=32) 19fda145e1053c5d36890e5169101beeb80875fea036c0b8de1c7bc66c427739,
                 CommitSig: (*btcec.Signature)(0xc0424fd340)({
                  R: (*big.Int)(0xc042196e60)(63724406601629180062774974542967536251589935445068131219452686511677818569431),
                  S: (*big.Int)(0xc042196e80)(18801056069249825825291287104931333862866033135609736119018462340006816851118)
                 }),
                 HtlcSigs: ([]*btcec.Signature) <nil>
                })
...

With this PR, it successfully ran 100 iterations without fail.

[Roasbeef]

LGTM 🕺🏾

Once I get your final PR merged in, we'll be able to switch over to using go1.9 in Travis!