Fix exit metric when there are no functions

[Luni-4]

No description provided.

[marco-c]

Same as #387 here

[codecov-io]

Codecov Report

Merging #388 (1601cc5) into master (2fff14d) will increase coverage by 0.33%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
+ Coverage   23.29%   23.62%   +0.33%     
==========================================
  Files          45       45              
  Lines        5491     5870     +379     
  Branches      805      826      +21     
==========================================
+ Hits         1279     1387     +108     
- Misses       3689     3947     +258     
- Partials      523      536      +13     

Impacted Files	Coverage Δ
src/macros.rs	28.12% <ø> (-23.23%)	⬇️
src/metrics/exit.rs	63.21% <75.00%> (+7.33%)	⬆️
src/c_macro.rs	66.15% <0.00%> (-14.98%)	⬇️
src/metrics/mi.rs	46.00% <0.00%> (-14.61%)	⬇️
src/metrics/halstead.rs	34.19% <0.00%> (-10.97%)	⬇️
src/metrics/loc.rs	47.40% <0.00%> (-6.57%)	⬇️
src/spaces.rs	56.28% <0.00%> (-5.56%)	⬇️
src/metrics/nom.rs	49.41% <0.00%> (-1.91%)	⬇️
src/languages/language_cpp.rs	0.00% <0.00%> (ø)
... and 16 more

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