Fix input_is_safe()

Hooray for unit tests!

src/tty.c

@@ -79,20 +79,24 @@ char *tty_input(pam_handle_t *pamh, const char *text, int echo_code)
 
 int input_is_safe(const char *input, size_t max_length)
 {
-    for (size_t i = 0; input[i]; i++)
-    {
-        if (i > max_length
-                /* Don't use isalnum() here because it is locale-dependent,
-                 * and don't use isalnum_l() because it is not portable.
-                 * Instead, hardcode a check for ASCII a-z A-Z 0-9. */
-                || !(  (input[i] >= 'a' && input[i] <= 'z')
-                    || (input[i] >= 'A' && input[i] <= 'Z')
-                    || (input[i] >= '0' && input[i] <= '9')))
-        {
-            return 0;
-        }
-    }
-    return 1;
+	size_t length = strlen(input);
+	if (length > max_length)
+	{
+		return 0;
+	}
+	for (size_t i = 0; i < length; i++)
+	{
+		/* Don't use isalnum() here because it is locale-dependent,
+		 * and don't use isalnum_l() because it is not portable.
+		 * Instead, hardcode a check for ASCII a-z A-Z 0-9. */
+		if (!(     (input[i] >= 'a' && input[i] <= 'z')
+			|| (input[i] >= 'A' && input[i] <= 'Z')
+			|| (input[i] >= '0' && input[i] <= '9')))
+		{
+			return 0;
+		}
+	}
+	return 1;
 }
 
 void tty_output(pam_handle_t *pamh, const char *text)