Fix use of dangling reference in wxFileSystemWatcher Unix code

Don't reference the "watch" object after possibly destroying it when handling
IN_DELETE_SELF inotify notification.

Closes #17122.

docs/changes.txt

@@ -107,6 +107,7 @@ wxGTK:
 - Improve wxSpinCtrl best size calculation.
 - Implement support for icon locations in wxMimeTypesManager (Hanmac).
 - Cosmetic fix for empty wxCheckBoxes display (Chuddah).
+- Fix crashes in wxFileSystemWatcher implementation (David Hart).
 
 wxMSW:
 

src/unix/fswatcher_inotify.cpp

@@ -385,6 +385,7 @@ protected:
             // if the wd isn't found: repeated IN_DELETE_SELFs can occur
             wxFileName fn = GetEventPath(watch, inevt);
             wxString path(fn.GetPathWithSep());
+            const wxString filespec(watch.GetFilespec());
 
             if (m_watchMap.erase(inevt.wd) == 1)
             {
@@ -406,7 +407,7 @@ protected:
 
             // Tell the owner, in case it's interested
             // If there's a filespec, assume he's not
-            if (watch.GetFilespec().empty())
+            if (filespec.empty())
             {
                 wxFileSystemWatcherEvent event(flags, fn, fn);
                 SendEvent(event);