A knife-wielding maniac ambushed a Queens teen just steps away from her door, slashing her 11 times before the bloodied girl’s mom came to the rescue, cops and witnesses said Tuesday. Natasha Martinez, 17, had finished her shift at McDonald’s and was just a few feet from her Woodhaven apartment when the madman pounced about 11:25 p.m. Monday. Hearing her daughter’s screams, Natasha’s mother, Wendy Martinez, came running outside—and the bloodthirsty attacker fled. …