[mythtv] minor fix for decoderbase.cpp

[Mark Weaver]

"mythfrontend -v all" tuned to an off-air DVB channel causes SIGSEGV 
without this.
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Index: decoderbase.cpp
===================================================================
--- decoderbase.cpp	(revision 7110)
+++ decoderbase.cpp	(working copy)
@@ -112,10 +112,12 @@
         PosMapEntry e = {it.key(), it.key() * keyframedist, it.data()};
         m_positionMap.push_back(e);
     }
-    VERBOSE(VB_PLAYBACK, QString("Position map filled from DB to: %1")
-            .arg((long int) m_positionMap[m_positionMap.size()-1].index));
+    if (!m_positionMap.empty())
+    {
+        VERBOSE(VB_PLAYBACK, QString("Position map filled from DB to: %1")
+                .arg((long int) m_positionMap[m_positionMap.size()-1].index));
+    }
 
-
     return true;
 }
 
@@ -180,8 +182,11 @@
         PosMapEntry e = {it.key(), it.key() * keyframedist, it.data()};
         m_positionMap.push_back(e);
     }
-    VERBOSE(VB_PLAYBACK, QString("Position map filled from Encoder to: %1")
-            .arg((long int) m_positionMap[m_positionMap.size()-1].index));
+    if (!m_positionMap.empty())
+    {
+        VERBOSE(VB_PLAYBACK, QString("Position map filled from Encoder to: %1")
+                .arg((long int) m_positionMap[m_positionMap.size()-1].index));
+    }
 
     return true;
 }