A maniac ignited a cup of liquid and hurled it at a stranger on a Manhattan subway Saturday, setting fire to the man’s shirt and burning his chest and neck, cops said. The terrifying episode is the second time in four months someone tossed flaming liquid at a straphanger — and authorities are investigating whether the same fire bug is to blame in both incidents. The latest incident unfolded as the No. 1 train was pulling into the Houston Street station at West Houston and Varick Streets around 2:45 p.m.