Fix EOF behavior in interactive mode with history

[michaelforney]

Previously, EOF would re-process the previous line, read one more
line, then exit. This is because bc_history_line left the buffer
untouched in case of EOF, and the bc_vm_stdin loop reads the next
line before checking the done condition of the previous iteration.

This fix also prevents reading past the end of the buffer if the
first user input is EOF, since in that case vec->len is still 0,
so (size_t)-1 is passed to bc_read_binary.

[codecov]

Codecov Report

Merging #6 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #6      +/-   ##
=========================================
+ Coverage    98.7%   98.7%   +<.01%     
=========================================
  Files          16      16              
  Lines        3478    3479       +1     
=========================================
+ Hits         3433    3434       +1     
  Misses         45      45

Impacted Files	Coverage Δ
src/vm.c	100% <100%> (ø)	⬆️

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[gavinhoward]

I am back from my vacation, so it's review time.

Let's see:

• Style? Style is correct.
• Is there a problem? Yes, there was, and it was one I didn't even know about.
• Does it fix the problem? Yes, it does.
• Does it pass the test suite? Yes, it does. (Actually, it doesn't pass when running the tests in parallel, but that is a bug in the test harnesses, not your changes.)

Result: Merged.

Thank you so much for the fix! I will add your name to the list of contributors.

[michaelforney]

Thanks!