It really makes no sense to check for a zero PC here. This function
is only colled from frame.c:get_prev_frame(), and there we already
deal with PC being zero.
The whole concept of using a zero PC as a marker for the end of the
frame chain is somewhat flawed. It prevents us from providing a
meaningful backtrace when the program has called a null function
pointer; see backtrace/1476. At the very least we will have to treat
a zero PC in the innermost differently. Classifying the a zero PC as
being inside the "main" function doesn't help. Therefore this patch
removes the first obstackle in fixing that PR.
Objections. Otherwise I'll commit this within a few days.