lib/strutils: fix strnlen() fallback

Addresses: https://github.com/karelzak/util-linux/issues/643
Signed-off-by: Karel Zak <kzak@redhat.com>

lib/strutils.c

@@ -243,11 +243,11 @@ void *mempcpy(void *restrict dest, const void *restrict src, size_t n)
 #ifndef HAVE_STRNLEN
 size_t strnlen(const char *s, size_t maxlen)
 {
-        int i;
+        size_t i;
 
         for (i = 0; i < maxlen; i++) {
                 if (s[i] == '\0')
-                        return i + 1;
+                        return i;
         }
         return maxlen;
 }