pingpong: fix response cache memcpy overflow

Response data for a handle with a large buffer might be cached and then
used with the "closure" handle when it has a smaller buffer and then the
larger cache will be copied and overflow the new smaller heap based
buffer.

Reported-by: Dario Weisser
CVE: CVE-2018-1000300
Bug: https://curl.haxx.se/docs/adv_2018-82c2.html