search: reset match state when selection is cancelled

While we’re in scrollback search mode, the selection may be
cancelled (for example, if the application is scrolling out the
selected text). Trying to e.g. extend the search selection after this
has happened triggered a crash.

This fixes it by simply resetting the search match state when the
selection is cancelled.

Closes #644

selection.c

@@ -21,6 +21,7 @@
 #include "grid.h"
 #include "misc.h"
 #include "render.h"
+#include "search.h"
 #include "uri.h"
 #include "util.h"
 #include "vt.h"
@@ -1058,6 +1059,8 @@ selection_cancel(struct terminal *term)
     term->selection.pivot.end = (struct coord){-1, -1};
     term->selection.direction = SELECTION_UNDIR;
     term->selection.ongoing = false;
+
+    search_selection_cancelled(term);
 }
 
 bool