usdt: Have Context::addsem_probe() nop if pid not specified
usdt: Have Context::addsem_probe() nop if pid not specified

This makes bcc_usdt_addsem*() more consistent with the bcc_usdt_enable*()
interface where if a USDT::Context was not constructed with a pid the
semaphore enablement nops.

brendangregg/bcc,tuxology/bcc,iovisor/bcc,tuxology/bcc,tuxology/bcc,brendangregg/bcc,tuxology/bcc,brendangregg/bcc,brendangregg/bcc,iovisor/bcc,iovisor/bcc,iovisor/bcc,brendangregg/bcc,tuxology/bcc,iovisor/bcc