Fix SIGSEGV due to writes to const char array.

If no console is configured, then the following code gets executed:
int conf_parse_config(void)
{
  ...
  char *fallback = FALLBACK_SHELL;
  ...
  tty_register(fallback);
  ...
}
int tty_register(char *line)
{
  ...
  cmd = strtok(line, " ");
  ...
}
FALLBACK_SHELL is constant character array and strtok tries to modify
the first argument, which causes a crash.

conf.c

@@ -519,10 +519,12 @@ int conf_parse_config(void)
 
 	result = parse_conf(FINIT_CONF);
 	if (!tty_num()) {
-		char *fallback = FALLBACK_SHELL;
+		char *fallback = NULL;
 
 		if (console)
 			fallback = console;
+		else
+			fallback = strdup(FALLBACK_SHELL);
 
 		tty_register(fallback);
 	}