CVE-2017-12989/RESP: Make sure resp_get_length() advances the pointer for invalid...

CVE-2017-12989/RESP: Make sure resp_get_length() advances the pointer for invalid lengths.

Make sure that it always sends *endp before returning and that, for
invalid lengths where we don't like a character in the length string,
what it sets *endp to is past the character in question, so we don't
run the risk of infinitely looping (or doing something else random) if a
character in the length is invalid.

This fixes an infinite loop discovered by Forcepoint's security
researchers Otto Airamo & Antti Levomäki.

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