A good Samaritan was pepper-sprayed and slashed this week when he got caught in the middle of a fight between dog owners in Harlem, cops and sources said. The 47-year-old victim stepped in when he spotted two dogs getting aggressive with each other, prompting their owners to start fighting around 8 p.m. Sunday on West 125th Street near Malcolm X Boulevard, law enforcement sources said.  A man and woman walking one of the dogs then ganged up on the do-gooder. One of the suspects pepper-sprayed