pipeline: Sanity check pipeline pointer before accessing it.

I got a crash with this stack:

curl/lib/url.c:2873 (Curl_removeHandleFromPipeline)
curl/lib/url.c:2919 (Curl_getoff_all_pipelines)
curl/lib/multi.c:561 (curl_multi_remove_handle)
curl/lib/url.c:415 (Curl_close)
curl/lib/easy.c:859 (curl_easy_cleanup)

Closes #704