ext/gd: Fix GH-13082

Issue occur when compiling with recent clang releases (> 13) and
with the '-Os' optimisation level, after using
`imageloadfont` which returns a proper GdFont cl

ext/gd: Fix GH-13082

Issue occur when compiling with recent clang releases (> 13) and
with the '-Os' optimisation level, after using
`imageloadfont` which returns a proper GdFont class leads to
a subtle bug when attempting to use via the imagefont* function.

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