[Bug go/65798] runtime.Caller returns ok=true when return data is invalid

[ian at airs dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65798

--- Comment #2 from Ian Lance Taylor <ian at airs dot com> ---
I don't see it as a good idea to ignore an empty file name, but I'm fine with
ignoring a 0 PC, so that is what I will implement.  Though I am definitely
curious how they got a 0 PC from runtime.Caller.