sd-journal: make sure sd_journal_add_match() also accepts SIZE_MAX as size

sd-journal: make sure sd_journal_add_match() also accepts SIZE_MAX as size

In many of our internal functions that take a pointer + a size we have
introduced the rule that SIZE_MAX as size means: take strlen().

sd_journal_add_match() has something similar, but the special value is
0, not SIZE_MAX. This is a bit ugly, since a zero size data block is
theoretically fine. The only reason sd_journal_add_match() gets away
with using this special value is because valid matches must consist of
at least 2 chars, hence cannot be zero.

But let's make this more robust and less surprising when compared to the
rest of our code, and *also* accept SIZE_MAX to mean strlen().

No actual code changes, just some clean-up.