builtin: Fix print crashing on nil "holes".

The engine implementation of `print` packs the varargs into a
table and passes the table directly to `table.concat`. If you
pass any value not supported by `table.concat` (particularly
`nil`), the server crashes. This is unexpected behavior, as
`print` is supposed to be able to work with anything.

This patch changes the implementation so it first converts
all arguments using `tostring`, which fixes the issue and
makes the custom `print` function compatible with the stock
Lua behavior.

builtin/init.lua

@@ -12,7 +12,11 @@ if core.print then
 	-- Override native print and use
 	-- terminal if that's turned on
 	function print(...)
-		core_print(table.concat({...}, "\t"))
+		local n, t = select("#", ...), { ... }
+		for i = 1, n do
+			t[i] = tostring(t[i])
+		end
+		core_print(table.concat(t, "\t"))
 	end
 	core.print = nil -- don't pollute our namespace
 end