Fix incorrect overflow guard condition in _ksba_ber_read_tl

* src/ber-help.c (_ksba_ber_read_tl): Remove useless extra overflow
  check.

The removed first condition is not necessary and could actually lead
to skipping the actual overflow check with the second condition. But
note that NHDR is always small in practice and thus this bug is not
immediately exploitable.

• GnuPG-bug-id: T8247
• Reported-by: Dominik Blain Dupuis
• Fixes-commit: 4b7d9cd4a018898d7714ce06f3faf2626c14582b