Due to the way we store muted topics, it is possible that a
muted topic stream name may no longer exist, and we need to
handle that case gracefully.
(imported from commit 4d18ec55e45213657a67e160848229678f212765)
Previously, we assumed that num_before or num_after would be always be non-zero
after adjustment for the anchor. However, we don't adjust num_before or
num_after when a narrow is specified.
(imported from commit 9239fef140e109b11bdfbeef42e9fbed78660ad1)
