libbb/login/su: do not sanitize shell name twice

function                                             old     new   delta
setup_environment                                    191     205     +14
login_main                                          1002     987     -15
su_main                                              474     458     -16
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/2 up/down: 14/-31)            Total: -17 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/loginutils/su.c b/loginutils/su.c
index db303af..72dd0f0 100644
--- a/loginutils/su.c
+++ b/loginutils/su.c
@@ -114,20 +114,14 @@
 		opt_shell = getenv("SHELL");
 	}
 
-	/* Make sure pw->pw_shell is non-NULL.  It may be NULL when NEW_USER
-	 * is a username that is retrieved via NIS (YP), that doesn't have
-	 * a default shell listed.  */
-	if (!pw->pw_shell || !pw->pw_shell[0])
-		pw->pw_shell = (char *)DEFAULT_SHELL;
-
 #if ENABLE_FEATURE_SU_CHECKS_SHELLS
-	if (opt_shell && cur_uid != 0 && restricted_shell(pw->pw_shell)) {
+	if (opt_shell && cur_uid != 0 && pw->pw_shell && restricted_shell(pw->pw_shell)) {
 		/* The user being su'd to has a nonstandard shell, and so is
 		 * probably a uucp account or has restricted access.  Don't
 		 * compromise the account by allowing access with a standard
 		 * shell.  */
 		bb_error_msg("using restricted shell");
-		opt_shell = NULL;
+		opt_shell = NULL; /* ignore -s PROG */
 	}
 	/* else: user can run whatever he wants via "su -s PROG USER".
 	 * This is safe since PROG is run under user's uid/gid. */