HTTP/2 packet corruption introduced by TS-4717 fix

We observed problems with large uploads over HTTP/2 via Firefox. Tracked it down to ATS sending a GOAWAY frame early because the frame appeared to be corrupted. The problem was introduced in the restructuring in TS-4717 to get rid of the unbounded recursion. The logic will return with one of two handlers set. One to deal with the case of starting to read a fresh frame and another to deal with the case of finishing reading a partially read frame. There is a path where we finish reading a frame but don't reset the handler to the new frame case. When the next data arrives the wrong handler starts reading it resulting in ATS thinking it has received a malformed frame.