Reduce memory leaks

[luolent]

Hello @wgtdkp This is a pull request to fix some of the memory leaks found in wtgcc, as reported by Valgrind. The fixes are quite simple and they follow your style. Even if the program frees all the memory after the program exits. I recommend this fix because it makes the code better and it can offer less troubles to someone else, who wants to use this project differently. Thanks!

[codecov[bot]]

Codecov Report

Merging #63 (e7fb79b) into master (c4cf781) will decrease coverage by 0.55%. The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   85.38%   84.82%   -0.56%     
==========================================
  Files          23       23              
  Lines        5076     5088      +12     
==========================================
- Hits         4334     4316      -18     
- Misses        742      772      +30     

Impacted Files	Coverage Δ
src/cpp.h	94.59% <ø> (-2.71%)	:arrow_down:
src/cpp.cc	81.91% <84.00%> (-3.62%)	:arrow_down:
src/main.cc	60.83% <100.00%> (+0.98%)	:arrow_up:
src/evaluator.cc	50.79% <0.00%> (-5.56%)	:arrow_down:
src/scanner.cc	93.18% <0.00%> (-0.33%)	:arrow_down:

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[wgtdkp]

@luolent Thanks for your contribution!

Looks like the code coverage is broken, would you mind take a look? https://github.com/wgtdkp/wgtcc/pull/63/checks?check_run_id=1787092052

[luolent]

Thanks @wgtdkp! Sure, I can take a look at it. How would you like me to improve the code coverage? Should I commit additional tests?

[wgtdkp]

@luolent I think some of your added code are not covered by the travis tests, see the warning here https://github.com/wgtdkp/wgtcc/pull/63/checks?check_run_id=1787092052. But actually, the original implementation is not tested either :)

Let's have this merged, thanks for your contribution!