packet: tests ignore errors in set up

[kortschak]

In #97 I found that the test was failing got go1.9 or lower, but not above. I tracked this down to a failure of the set up to provide an non-empty input string because hex.DecodeString was given an odd-length string. The call to hex.DecodeString in the tests in "io_test.go" ignore the "error" returned allowing this kind of test corruption.