x509: remove dead call to strlen()

The condition `userlen == -1` isn't possible because this is already checked
on line 159 above and the subsequent strlen(3) call guarantees that it's value
is positive.

Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
(Merged from #16987)

crypto/x509/v3_sxnet.c

@@ -177,8 +177,6 @@ int SXNET_add_id_INTEGER(SXNET **psx, ASN1_INTEGER *zone, const char *user,
 
     if ((id = SXNETID_new()) == NULL)
         goto err;
-    if (userlen == -1)
-        userlen = strlen(user);
 
     if (!ASN1_OCTET_STRING_set(id->user, (const unsigned char *)user, userlen))
         goto err;