CVE-2017-11541: In safeputs(), check the length before checking for a NUL terminator.

CVE-2017-11541: In safeputs(), check the length before checking for a NUL terminator.

safeputs() doesn't do packet bounds checking of its own; it assumes that
the caller has checked the availability in the packet data of all maxlen
bytes of data.  This means we should check that we're within the
specified limit before looking at the byte.

This fixes a buffer over-read discovered by Kamil Frankowicz.

Add a test using the capture file supplied by the reporter(s).