BUG/MINOR: spoe: Fix condition to abort processing on client abort

BUG/MINOR: spoe: Fix condition to abort processing on client abort

The test to detect client aborts in the SPOE, introduced by commit b3be3b94a
("BUG/MEDIUM: spoe: Properly abort processing on client abort"), was no
correct. Producer flags must not be tested. Only the frontend SC must be
tested when the abortonclose option is set.

Because of this bug, when a client aborted, the SPOE processing was aborted
too, regardless the abortonclose option.

This patch must be backpoeted with the commit above, so as far as 3.1.