mame: Use an intermediate variable in get_supported_games()

This forces Vala->C translator to introduce an additional variable for the
array length instead of recomputing it on every iteration.
4 jobs for master in 25 minutes and 4 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #102029
flatpak:app

00:13:29

 
  Test
passed #102030
flatpak:tests

00:05:20

 
  Cross Distro
passed #102031
fedora:29

00:02:21

passed #102032
ubuntu:devel

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