faulthandler's dump_traceback doesn't handle case where top-most frame is FRAME_OWNED_BY_CSTACK

[colesbury]

Bug report

We properly skip trampoline frames when they are not the top-most frame:

cpython/Python/traceback.c

Lines 979 to 982 in 3bd942f

	if (frame->owner == FRAME_OWNED_BY_CSTACK) {
	/* Trampoline frame */
	frame = frame->previous;
	}

But if tstate->current_frame is a trampoline frame (i.e., FRAME_OWNED_BY_CSTACK) then dump_traceback will crash if faulthandler is triggered when executing a trampoline frame.