Don't crash when LANG is not set

--- lletters-0.1.95/lln.c	2020-10-03 20:22:43.470743688 +0100
+++ lletters-0.1.95/lln.c	2020-10-03 20:31:03.322335396 +0100
@@ -138,7 +138,8 @@ main (int argc, char *argv[])
   /* End change by Gopal Narayanan */
 
   /* Change by Lalo Martins, Feb 27 1999: */
-  lang = strtok(getenv ("LANG"), "_");
+  char* envlang = getenv("LANG");
+  lang = strtok(envlang ? envlang : "", "_");
 //  lang = strtok(lang, "_");
 
   if (!lang)