passwd: fix crashes for unencrypted passwords starting with 'x'.

When deciding where the previous hash should come from, is is
assumed that 'x' started strings all mean to look in shadow.
This is probably harmless in practice, since modern Linux still
use only hashes instead of raw passwords.
However, this is more robust, and more importantly, it is more
consistent with the previous check, which explicitly tests for
the string to be "x".

passwd.c

@@ -210,7 +210,8 @@ main(int argc, char *argv[])
 		if (pw->pw_passwd[0] == '\0') {
 			goto newpass;
 		}
-		if (pw->pw_passwd[0] == 'x')
+		if (pw->pw_passwd[0] == 'x' &&
+		    pw->pw_passwd[1] == '\0')
 			prevhash = spw->sp_pwdp;
 		else
 			prevhash = pw->pw_passwd;