Fix lfs.attributes and lfs.symlinkattributes extra argument handling

When the second argument is not a string, _file_info() wants to
ensure that there is a table on top of the stack: the second argument
or a new table. If a new table is pushed it's created on top immediately,
but if a table is passed as the second argument it can be followed
by extra arguments, with the last one ending up being used as a table,
causing a crash. The fix is to remove any potential extra arguments
using `lua_settop(L, 2)`.

Also added a few tests for this case. Ref #80.

src/lfs.c

@@ -827,7 +827,8 @@ static int _file_info_ (lua_State *L, int (*st)(const char*, STAT_STRUCT*)) {
                 /* member not found */
                 return luaL_error(L, "invalid attribute name '%s'", member);
         }
-        /* creates a table if none is given */
+        /* creates a table if none is given, removes extra arguments */
+        lua_settop(L, 2);
         if (!lua_istable (L, 2)) {
                 lua_newtable (L);
         }

tests/test.lua

@@ -132,6 +132,17 @@ for key, value in pairs(attr) do
           "lfs.attributes values not consistent")
 end
 
+-- Check that lfs.attributes accepts a table as second argument
+local attr2 = {}
+lfs.attributes(tmpfile, attr2)
+for key, value in pairs(attr2) do
+  assert (value == lfs.attributes (tmpfile, key),
+          "lfs.attributes values with table argument not consistent")
+end
+
+-- Check that extra arguments are ignored
+lfs.attributes(tmpfile, attr2, nil)
+
 -- Remove new file and directory
 assert (os.remove (tmpfile), "could not remove new file")
 assert (lfs.rmdir (tmpdir), "could not remove new directory")