curl_easy_getopt: Handle API violation gracefully

This fixes a NULL dereference in the case where the client asks for
CURLINFO_TLS_SESSION data after the (TLS) session has already been
destroyed (i.e. curl_easy_perform has already completed for this
handle). Instead of crashing, we now return a CURLSSLBACKEND_NONE
error.
multi.c: Fixed compilation error introduced in commit a900d45489fc14

Systems that define SIGPIPE_VARIABLE as a noop would not compile as
restore_pipe was defined afterwards.