[Backtracing] Fix crash in CrashAsync test on Ubuntu 18.04.

This seems to be happening because we're hitting an invalid frame
on the stack where the program counter value is read back as zero.

If we find one of those, treat it as the end of our stack walk.

rdar://110846253

Author: al45tair

stdlib/public/Backtracing/FramePointerUnwinder.swift

@@ -161,7 +161,7 @@ public struct FramePointerUnwinder<C: Context, M: MemoryReader>: Sequence, Itera
           return nil
         }
 
-        if next <= fp {
+        if next <= fp || pc == 0 {
           return nil
         }