Time for preview max 10 minutes into the program

The time of the preview is 1/3 of the total program duration
limited to 10 minutes after the start of the program.
This limit is added to prevent accidentally revealing the
result of sport events in long recordings.

(cherry picked from commit b571d81)
Signed-off-by: Klaas de Waal <klaas@kldo.nl>

mythtv/libs/libmythtv/previewgenerator.cpp

@@ -677,7 +677,10 @@ bool PreviewGenerator::LocalPreviewRun(void)
         }
         if (programDuration > 0)
         {
-            captime = startEarly + (programDuration / 3);
+            captime = programDuration / 3;
+            if (captime > 600)
+                captime = 600;
+            captime += startEarly;
         }
         if (captime < 0)
             captime = 600;