[lldb] Prevent underflow in crashlog.py

Avoid a OverflowError (an underflow really) when the pc is zero. This
can happen for "unknown frames" where the crashlog generator reports a
zero pc. We could omit them altogether, but if they're part of the
crashlog it seems fair to display them in lldb as well.

rdar://92686666

Differential revision: https://reviews.llvm.org/D125716