Correct parse os option -f

[jcbf]

Fix for #62

[codecov]

Codecov Report

Merging #63 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #63   +/-   ##
=======================================
  Coverage   86.81%   86.81%           
=======================================
  Files           1        1           
  Lines         508      508           
=======================================
  Hits          441      441           
  Misses         67       67

Impacted Files	Coverage Δ
smf-spf.c	86.81% <100%> (ø)	⬆️

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[coveralls]

Coverage remained the same at 86.965% when pulling 9f9f12c on #62_opt_f_ignored into 479ae56 on master.