Check for null frame buffers in commflag

Fixes #9304.  I ran this several times against the example bad recording, and
it gets very spammy with "Unknown decoding errors", but carries on to
completion every time, no crash.  Please reopen a new bug if this doesn't fix
the issue.
(cherry picked from commit 7338bd9)

mythtv/programs/mythcommflag/ClassicCommDetector.cpp

@@ -740,7 +740,7 @@ void ClassicCommDetector::ProcessFrame(VideoFrame *frame,
     int rightDarkCol = width - commDetectBorder - 1;
     FrameInfoEntry fInfo;
 
-    if (!frame || frame_number == -1 || frame->codec != FMT_YV12)
+    if (!frame || !(frame->buf) || frame_number == -1 || frame->codec != FMT_YV12)
     {
         VERBOSE(VB_COMMFLAG, "CommDetect: Invalid video frame or codec, "
                 "unable to process frame.");