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.
6 jobs for wip/exalm/performance in 12 minutes and 25 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #102003
flatpak:app

00:02:24

 
  Test
passed #102004
flatpak:tests

00:05:41

 
  Review
passed #102005
review

00:00:22

passed #102006
manual
stop_review

00:00:20

 
  Cross Distro
passed #102007
fedora:29

00:03:37

passed #102008
ubuntu:devel

00:02:13