Fix bug in wxFileName::Normalize() with leading ".."

Don't discard initial ".." when there is more than one such component in
the beginning of the path.

Closes #18800.

src/common/filename.cpp

@@ -1518,13 +1518,16 @@ bool wxFileName::Normalize(int flags,
                         continue;
 
                 }
-                else // Normal case, go one step up.
+                else // Normal case, go one step up unless it's .. as well.
+                {
+                    if (m_dirs.back() != wxT("..") )
                     {
                         m_dirs.pop_back();
                         continue;
                     }
                 }
             }
+        }
 
         m_dirs.Add(dir);
     }

tests/filename/filenametest.cpp

@@ -355,6 +355,7 @@ void FileNameTestCase::TestNormalize()
         { "b/../bar", wxPATH_NORM_DOTS, "bar", wxPATH_UNIX },
         { "c/../../quux", wxPATH_NORM_DOTS, "../quux", wxPATH_UNIX },
         { "/c/../../quux", wxPATH_NORM_DOTS, "/quux", wxPATH_UNIX },
+        { "../../quux", wxPATH_NORM_DOTS, "../../quux", wxPATH_UNIX },
 
         // test wxPATH_NORM_TILDE: notice that ~ is only interpreted specially
         // when it is the first character in the file name