userspace/libscap: handle trace file with no events

Currently when you save a file with 0 events, you receive a "expecting 8 bytes, read 0 at /sysdig/userspace/libscap/scap_savefile.c, line 1662. Is the file truncated?"

This patch tries to give the user a more meaningful message, by checking if the header is the only one to be not there when you read 0 bytes.