[PLAT-6059] Fix empty traces in some edge cases

[kstenerud]

Goal

The code was incorrectly returning 0 stack trace entries in some error edge cases when it should have been returning the current count. This is a rare edge case, but it may have been contributing to our CI flakes.

Testing

Reran all tests.

[github-actions]

Bugsnag.framework binary size did not change - 815,960 bytes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +4  +0.0%      +4    __TEXT,__text
  -0.0%      -4  -0.0%      -4    [__TEXT]
  [ = ]       0  [ = ]       0    TOTAL

Generated by 🚫 Danger

[nickdowell]

This does appear to fix the issue!

https://buildkite.com/bugsnag/bugsnag-cocoa/builds/5920