Fix panic when loading composite fonts

[adrg]

Related to #87

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This change is 

[codecov]

Codecov Report

Merging #164 into development will increase coverage by 0.01%.
The diff coverage is 11.11%.

@@               Coverage Diff               @@
##           development     #164      +/-   ##
===============================================
+ Coverage        57.54%   57.55%   +0.01%     
===============================================
  Files              189      189              
  Lines            34414    34412       -2     
===============================================
+ Hits             19802    19805       +3     
+ Misses           12067    12059       -8     
- Partials          2545     2548       +3

Impacted Files	Coverage Δ
creator/styled_paragraph.go	87.5% <0%> (-0.59%)	⬇️
model/internal/fonts/ttfparser.go	68% <0%> (-0.31%)	⬇️
creator/paragraph.go	83.01% <25%> (-0.08%)	⬇️
model/font.go	57.11% <0%> (-0.36%)	⬇️
internal/textencoding/truetype.go	18% <0%> (+4%)	⬆️
internal/textencoding/encoder.go	26.47% <0%> (+26.47%)	⬆️

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[gunnsth]

LGTM