lua: make the map command check its arguments

Fixes #147.
3 jobs for lua-map-crash in 1 minute and 37 seconds
Status Job ID Name Coverage
  Build
passed #2207
docker linux amd64
build:linux:amd64

00:00:29

 
  Test
passed #2209
docker linux amd64
deckard:linux:amd64

00:01:08

passed #2208
docker linux amd64
test:linux:amd64

00:00:32