fix: address open PR #105 review comments (leaks + null safety)

- ui/window.c win_show_subwin (#1144): unconditional newpad without
  freeing the previous pad leaks the ncurses subwin if the function
  is invoked twice without an intervening win_hide_subwin. Added a
  delwin guard.
- omemo/omemo.c omemo_on_disconnect (#1126): hash tables and
  libsignal handles are destroyed unconditionally even when a full
  init never ran (e.g. shutdown after a failed load), and neither
  g_hash_table_destroy nor signal_*_destroy are documented as
  NULL-safe. NULL-check each handle before tearing it down.
- xmpp/roster_list.c (#1140 / #1141 / #1160): null the caller's
  pointer immediately after resource_destroy() so a stale reference
  cannot trigger a double-free; resource_destroy itself only zeroes
  the struct fields it owns and cannot reach back to the caller.

src/omemo/omemo.c

@@ -281,11 +281,24 @@ omemo_on_disconnect(void)
     }
 
     free_keyfile(&omemo_ctx.identity);
+    /* These can be NULL when omemo_on_disconnect runs before a full
+     * init has populated them; g_hash_table_destroy and the libsignal
+     * destructors are not documented to be NULL-safe. */
+    if (omemo_ctx.known_devices) {
         g_hash_table_destroy(omemo_ctx.known_devices);
+    }
+    if (omemo_ctx.device_list_handler) {
         g_hash_table_destroy(omemo_ctx.device_list_handler);
+    }
+    if (omemo_ctx.device_list) {
         g_hash_table_destroy(omemo_ctx.device_list);
+    }
+    if (omemo_ctx.store) {
         signal_protocol_store_context_destroy(omemo_ctx.store);
+    }
+    if (omemo_ctx.signal) {
         signal_context_destroy(omemo_ctx.signal);
+    }
     memset(&omemo_ctx, 0, sizeof(omemo_ctx));
 
     wins_omemo_trust_changed(NULL);

src/ui/window.c

@@ -577,6 +577,11 @@ win_show_subwin(ProfWin* window)
     }
 
     ProfLayoutSplit* layout = (ProfLayoutSplit*)window->layout;
+    /* Free a previous pad if win_show_subwin was called twice in a row
+     * without an intervening win_hide_subwin. */
+    if (layout->subwin) {
+        delwin(layout->subwin);
+    }
     layout->subwin = newpad(PAD_MIN_HEIGHT, subwin_cols);
     wbkgd(layout->subwin, theme_attrs(THEME_TEXT));
     wresize(layout->base.win, PAD_MIN_HEIGHT, cols - subwin_cols);

src/xmpp/roster_list.c

@@ -88,6 +88,7 @@ _pendingPresence_free(ProfPendingPresence* presence)
         g_date_time_unref(presence->last_activity);
     free(presence->barejid);
     resource_destroy(presence->resource);
+    presence->resource = NULL;
     free(presence);
 }
 
@@ -136,6 +137,7 @@ roster_update_presence(const char* const barejid, Resource* resource, GDateTime*
     if (contact == NULL) {
         /* Don't lose resource when there is no owner. */
         resource_destroy(resource);
+        resource = NULL;
         return FALSE;
     }
     if (!_datetimes_equal(p_contact_last_activity(contact), last_activity)) {