attribcheck: improve robustness of warn function

Author: tarleb

src/resources/filters/modules/attribcheck.lua

@@ -136,10 +136,13 @@ local BlockTypes = {
 
 local function warn_conversion (expected, actual, shift, extrainfo)
   local dbginfo = debug.getinfo(5 + (shift or 0), 'Sln')
-  warn(actual .. ' instead of ' .. expected .. ': ' ..
-       (dbginfo.name or '<no name>') .. ' in ' .. dbginfo.source .. ':' ..
-       dbginfo.currentline ..
-       (extrainfo and '\n' .. extrainfo or ''))
+  local dbginfostr = 'no detailed debug info available'
+  if dbginfo then
+    dbginfostr = (dbginfo.name or '<no name>') .. ' in ' .. dbginfo.source
+      .. ':' .. dbginfo.currentline
+  end
+  warn(actual .. ' instead of ' .. expected .. ': ' .. dbginfostr
+       .. (extrainfo and '\n' .. extrainfo or ''))
   return
 end