src/lib/interpreter: fix invalid size of args array for lua_subprocess

This also implements one TODO to validate type.
6 jobs for fix-subprocess in 3 minutes and 57 seconds (queued for 8 seconds)
Status Name Job ID Coverage
  Test
passed cppcheck #321733

00:00:30

passed luacheck #321734

00:00:29

passed system-tests #321731

00:00:59

passed system-valgrind-tests #321732

00:03:57

passed unit-tests #321729

00:00:57

passed unit-valgrind-tests #321730

00:03:12