logging: Also protect gpgrt_inc_errorcount against counter overflow.

* src/logging.c (_gpgrt_inc_errorcount): Protect against counter
overflow.
(_gpgrt_logv_internal): Use that function here so that we have only
one check for counter overflow.

We already had an overflow checking for log_error but not for the
silent increment function.

• Signed-off-by: Werner Koch <wk@gnupg.org>