Fix potential buffer overflow in PS_ReadPrimitive

Found by Coverity.

code/botlib/l_script.c

@@ -828,7 +828,7 @@ int PS_ReadPrimitive(script_t *script, token_t *token)
 	len = 0;
 	while(*script->script_p > ' ' && *script->script_p != ';')
 	{
-		if (len >= MAX_TOKEN)
+		if (len >= MAX_TOKEN - 1)
 		{
 			ScriptError(script, "primitive token longer than MAX_TOKEN = %d", MAX_TOKEN);
 			return 0;