Fix #584

[st0012]

The cause of this issue is that Goby pops the incorrect call frame when evaluating a break statement inside a built-in method's block.

When evaluating a built-in method's call frame, Goby should pop it after the evaluation is finished. And we do this by calling t.callFrameStack.pop() directly, which makes sense in most cases.

However, because the "break" instruction pops out 3 call frames every time (which is necessary, please read the comments inside Break instruction for more detail), it'd pop the built-in method's frame in advance. So when Goby finishes the built-in frame's evaluation later and tries to pop the frame from the stack, it actually pops the top frame, which later causes the nil pointer error.

[codecov]

Codecov Report

Merging #837 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #837   +/-   ##
=======================================
  Coverage   81.08%   81.08%           
=======================================
  Files          54       54           
  Lines        7433     7434    +1     
=======================================
+ Hits         6027     6028    +1     
  Misses       1182     1182           
  Partials      224      224           

Impacted Files	Coverage Δ
vm/thread.go	90.45% <100.00%> (+0.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c21b38e...9c2e449. Read the comment docs.