disobedience/choose-search.c: Fix segfault when search terms change.

disobedience/choose-search.c: Fix segfault when search terms change.

The observed problem:  Sometimes, when typing search terms into the box,
DisObedience crashes.  This seems to happen more often when the server
is slow to respond.

Here's what's going on.  Suppose that a search finds some results.  Then
`choose_make_visible' tries to expand the tree to display them.  If some
of the results aren't populated in the tree yet, then
`choose_make_one_visible' arranges to fetch the necessary information
from the server and informs its caller, `choose_make_visible', that
it'll have to try again later.  When this happens, `choose_make_visible'
hooks itself onto the `choose-more-tracks' event and waits for the
information to arrive.

Suppose that, while this is happening, the user edits the search terms;
a new search starts, but this one returns no results.  Then
`choose_search_completed' zaps the `choose_search_results' vector and
returns.

Now the metadata from the old search arrives from the server;
`choose_make_visible' is called back.  There are now no search results,
so it thinks its work is done and tries to scroll the list widget so
that the first result is visible -- but the result vector has been
clobbered.  Result: misery.

To fix this, cancel the `choose-more-tracks' event handler when a new
search finishes.  Now we can't get into this situation any more.