Fix pointer check when changing password

Fix pointer check when changing password

There was a missing pointer derefernce when checking for repeated
password in a password change dialog. This mistake was spotted by
GCC 7:

shigofumi.c: In function 'do_passwd':
shigofumi.c:1471:45: error: comparison between pointer and zero character constant [-Werror=pointer-compare]
         if (!new_password2 || new_password2 == '\0') {
                                             ^~
shigofumi.c:1471:31: note: did you mean to dereference the pointer?
         if (!new_password2 || new_password2 == '\0') {

This patch fixes it.