in crypt-sha*, reject excessive rounds as error rather than clamping

the reference implementation clamps rounds to [1000,999999999]. we
further limited rounds to at most 9999999 as a defense against extreme
run times, but wrongly clamped instead of treating out-of-bounds
values as an error, thereby producing implementation-specific hash
results. fixing this should not break anything since values of rounds
this high are not useful anyway.

src/crypt/crypt_sha256.c

@@ -230,7 +230,7 @@ static char *sha256crypt(const char *key, const char *setting, char *output)
 		if (u < ROUNDS_MIN)
 			r = ROUNDS_MIN;
 		else if (u > ROUNDS_MAX)
-			r = ROUNDS_MAX;
+			return 0;
 		else
 			r = u;
 		/* needed when rounds is zero prefixed or out of bounds */

src/crypt/crypt_sha512.c

@@ -252,7 +252,7 @@ static char *sha512crypt(const char *key, const char *setting, char *output)
 		if (u < ROUNDS_MIN)
 			r = ROUNDS_MIN;
 		else if (u > ROUNDS_MAX)
-			r = ROUNDS_MAX;
+			return 0;
 		else
 			r = u;
 		/* needed when rounds is zero prefixed or out of bounds */