BUG/MEDIUM: config: ignore empty args in skipped blocks

BUG/MEDIUM: config: ignore empty args in skipped blocks

As returned by Christian Ruppert in GH issue #3203, we're having an
issue with checks for empty args in skipped blocks: the check is
performed after the line is tokenized, without considering the case
where it's disabled due to outer false .if/.else conditions. Because
of this, a test like this one:

    .if defined(SRV1_ADDR)
        server srv1 "$SRV1_ADDR"
    .endif

will fail when SRV1_ADDR is empty or not set, saying that this will
result in an empty arg on the line.

The solution consists in postponing this check after the conditions
evaluation so that disabled lines are already skipped. And for this
to be possible, we need to move "errptr" one level above so that it
remains accessible there.

This will need to be backported to 3.3 and wherever commit 1968731765
("BUG/MEDIUM: config: solve the empty argument problem again") is
backported. As such it is also related to GH issue #2367.