baseparse: Don't forget error returns when processing more

baseparse: Don't forget error returns when processing more

If parsing returns a non-OK flow return in the middle
of processing an input buffer, don't overwrite that
if a later return is OK again - the subclass might
return not-linked in the middle, and then discard
subsequent data without pushing while returning OK.

A later success doesn't invalidate the earlier failure,
but we should continue processing after not-linked, so
as to keep parse state consistent.

https://bugzilla.gnome.org/show_bug.cgi?id=779831