Initialize the MythUIImage filename correctly

Fixes #9634.  Fixes #9708.  Refs #9547.

If the filename does not start with "/", "http://", "https://" or "ftp://", then
the actual filename loaded is uninitialized.  It can be any random crap on the
stack.  I'm honestly surprised this worked at all :)

This may fix things for #9547 as well, and should be tested with that
scenario too.

(cherry picked from commit 2c7735e)

mythtv/libs/libmythui/mythuiimage.cpp

@@ -87,7 +87,7 @@ class ImageLoadThread : public QRunnable
     void run()
     {
         threadRegister("ImageLoad");
-        QString tmpFilename;
+        QString tmpFilename = m_filename;
         if ((m_filename.startsWith("/")) ||
             (m_filename.startsWith("http://")) ||
             (m_filename.startsWith("https://")) ||