Code optimizations NULL checks

[selvamKrish]

As part of this code optimization,

• Added NULL checks before doing any string operation and free
• Replaced malloc with strdup

[codecov]

Codecov Report

Merging #48 into master will decrease coverage by 1.99%.
The diff coverage is 34.9%.

@@           Coverage Diff           @@
##           master      #48   +/-   ##
=======================================
- Coverage   90.41%   88.41%   -2%     
=======================================
  Files           3        3           
  Lines        1888     1917   +29     
  Branches      201      226   +25     
=======================================
- Hits         1707     1695   -12     
- Misses         57       74   +17     
- Partials      124      148   +24

Impacted Files	Coverage Δ
tests/simple.c	93.39% <100%> (-0.04%)	⬇️
src/wdmp_internal.c	80.3% <33.33%> (-5.94%)	⬇️
src/wdmp-c.c	73.03% <8.33%> (-4.68%)	⬇️

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