lib/encrypt.c: Do not exit in error case

lib/encrypt.c: Do not exit in error case

If crypt fails, pw_encrypt calls exit. This has the consequence that the
plaintext password is not cleared.

A valid password can fail if the underlying library does not support it.
One such example is SHA512, for which the password must not be longer
than 256 characters on musl. A password longer than this with glibc
works, so it is actually possible that a user, running passwd, tries to
enter the old password but the musl-based passwd binary simply exits.
Let passwd clear the password before exiting.

Reviewed-by: Alejandro Colomar <alx@kernel.org>
Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>