Fix dereferencing invalid iterator in wxMemoryFSHandlerBase

Don't assume that m_findIter is always valid initially, i.e. after
FindFirst() is called, as this is not the case if the memory FS is
empty, i.e. which no files had been added to it yet.

This also allows to simplify the iteration logic, as we don't need to
check m_findArgument (which, in turn, obviates the need to clear it when
we reach the end of the iteration) and can just use m_findIter directly.

Closes #18416.

src/common/fs_mem.cpp

@@ -147,22 +147,16 @@ wxString wxMemoryFSHandlerBase::FindFirst(const wxString& url, int flags)
 
 wxString wxMemoryFSHandlerBase::FindNext()
 {
-    // m_findArgument is used to indicate that search is in progress, we reset
-    // it to empty string after iterating over all elements
-    while ( !m_findArgument.empty() )
+    while ( m_findIter != m_Hash.end() )
     {
-        // test for the match before (possibly) clearing m_findArgument below
-        const bool found = m_findIter->first.Matches(m_findArgument);
+        const wxString& path = m_findIter->first;
 
         // advance m_findIter first as we need to do it anyhow, whether it
         // matches or not
-        const wxMemoryFSHash::const_iterator current = m_findIter;
+        ++m_findIter;
 
-        if ( ++m_findIter == m_Hash.end() )
-            m_findArgument.clear();
-
-        if ( found )
-            return "memory:" + current->first;
+        if ( path.Matches(m_findArgument) )
+            return "memory:" + path;
     }
 
     return wxString();