list_cursor: Avoid tracebacks.
list_cursor: Avoid tracebacks.

In rare situations we would get tracebacks from
list_cursor on the line that I changed here.  We
went the entire month of May without a traceback
here, and I can't reproduce the problem.

This is a pretty clear fix, though, and it will
hopefully lead to a more enlightening symptom.

The likely scenario here is that you use `q` to
navigate the stream list and then unsubscribe.
I tested that and couldn't get a traceback,
but I do think the traceback indicates some
possible issues.

The behavior I saw when I did this
appeared to be mostly harmless.
When I deleted a row (by unsubscribing), the code
seemed to effectively disable the cursor.  It's
possible we should go to the next row or fully disable
the search.

I opened #15439 to follow up on this and other
cursor-related issues.

showell/zulip,showell/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,synicalsyntax/zulip,kou/zulip,timabbott/zulip,zulip/zulip,rht/zulip,brainwane/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,timabbott/zulip,timabbott/zulip,punchagan/zulip,punchagan/zulip,timabbott/zulip,kou/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,brainwane/zulip,synicalsyntax/zulip,brainwane/zulip,eeshangarg/zulip,synicalsyntax/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,timabbott/zulip,brainwane/zulip,punchagan/zulip,punchagan/zulip,andersk/zulip,kou/zulip,brainwane/zulip,andersk/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,showell/zulip,synicalsyntax/zulip,eeshangarg/zulip,synicalsyntax/zulip,rht/zulip,rht/zulip,showell/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,brainwane/zulip,timabbott/zulip,hackerkid/zulip,timabbott/zulip,synicalsyntax/zulip,hackerkid/zulip,brainwane/zulip,showell/zulip,kou/zulip,eeshangarg/zulip,showell/zulip,hackerkid/zulip,kou/zulip,kou/zulip,kou/zulip