http: restore buffer pointer when bad response-line is parsed

... leaving the k->str could lead to buffer over-reads later on.

CVE: CVE-2018-1000301
Assisted-by: Max Dymond

Detected by OSS-Fuzz.
Bug: https://curl.haxx.se/docs/adv_2018-b138.html
Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=7105