When mentioning a user, the markup post-processor did not handle the
case where the mentioned user did not exist well: it tried to skip to
the next node, which in turn, ended up skipping the rest of the line.
To fix this, lets skip just the mentioned, but non-existing user, and
continue processing the current node from there.
Fixes#3535.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
(cherry picked from commit 9a01062ae248327e62c478fd2ec5bd7518ea4f0b)
75aa4c28f0