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 Job ID Name Coverage
  Test
passed #321733
cppcheck

00:00:30

passed #321734
luacheck

00:00:29

passed #321731
system-tests

00:00:59

passed #321732
system-valgrind-tests

00:03:57

passed #321729
unit-tests

00:00:57

passed #321730
unit-valgrind-tests

00:03:12