lib/chkhash.c: is_valid_hash(): Accept an empty hash

It represents a passwordless account.
That is discouraged, but accepted.

Link: <https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1124835>
Reported-by: Marc 'Zugschlus' Haber <[email protected]>
Reported-by: "Serge E. Hallyn" <[email protected]>
Signed-off-by: Alejandro Colomar <[email protected]>

Author: alejandro-colomar

lib/chkhash.c

@@ -42,6 +42,9 @@ is_valid_hash(const char *hash)
 {
 	hash = strprefix(hash, "!") ?: hash;
 
+	if (streq(hash, ""))
+		return true;  // Passwordless account; discouraged
+
 	if (streq(hash, "*"))
 		return true;