Comedian Heather McDonald suffered a fractured skull in her fall onstage during a set in Arizona over the weekend, her representative said. Medical staff began a “comprehensive work up” on McDonald when she was hospitalized Saturday after her show ended early in Tempe, the rep told People. “Thus far the test have revealed no underlying medical issues that may have precipitated this event,” the representative said. McDonald, 51, previously said in an Instagram video that she collapsed toward the beginning of Saturday’s performance. “I got up, I did one joke, and I felt so dizzy,” McDonald said….