geanylua: Fix a few warnings and potential crashes

[b4n]

⚠️ Beware, this is entirely untested, beside compiling OK.

[xiota]

Looks good to merge. Tested with examples/work/01.edit-test-script.lua and following scripts:

-- script 1
local files={}

for file in geany.documents()
do
  table.insert(files,file)
end

if ( #files > 0 )
then
  file=geany.choose("Choose a tab to activate:", files)
  if (file)
  then
    geany.save(file)
    geany.activate(file)
  end
else
  geany.message("No files!")
end

-- script 2
if geany.save(-5) then
  geany.status('save document -5')
else
  geany.status('save document 2')
  geany.save(2)
end

if geany.activate(-3) then
  geany.status('activate third tab')
else
  geany.status('activate fourth document')
  geany.activate(4)
end

Script 2 outputs following in status window:

00:00:00: save document 2
00:00:00: File ... saved.
00:00:00: activate third tab

[xiota]

Also, confirmed crash when unpatched and fixed when patched:

geany.save('/tmp/does-not-exist.txt')