Street vendors be warned: Take a bathroom break, and it may be your business that gets flushed down the toilet. That’s what happened to Shiraj Islam, a nut vendor for 12 years, who suddenly found himself without a livelihood. Health inspectors, cracking down on unattended vendor carts across the city, revoked his permit last week when a bathroom emergency forced him to leave his stand in lower Manhattan. “Everybody has to go sometimes,” Islam, 42, who had been a fixture near J&R Music World, told The Post.