UniFirst Corp., a Wilmington-based uniform company, posted first-quarter results that topped Wall Street expectations. Net income was $30.8 million, or $1.54 per share, up from $25.8 million, or $1.30 per share, a year earlier. It also disclosed it received a letter from New England Compounding Center, asking it to take legal responsibility for claims against the pharmacy for tainted drugs blamed for a deadly meningitis outbreak. UniFirst said a subsidiary helped clean portions of the pharmacy’s cleanroom facility, but maintained its services were limited and it was not responsible for the tainted drugs.