The documentation for this function says that the smartlist can
contain NULLs, but the code only handled NULLs if they were at the
start of the list.

We didn't notice this for a long time, because when Tor is run
normally, the sequence of msg_id_t is densely packed, and so this
list (mapping msg_id_t to channel_id_t) contains no NULL elements.
We could only run into this bug:
  * when Tor was running in embedded mode, and starting more than once.
  * when Tor ran first with more pubsub messages enabled, and then
    later with fewer.
  * When the second run (the one with fewer enabled pubsub messages)
    had at least some messages enabled, and those messages were not
    the ones with numerically highest msg_id_t values.

Fixes bug 31898; bugfix on 47de9c7b
in 0.4.1.1-alpha.