Fix AdaptationFieldSize function

Use the data() function to get a pointer to access the fifth element of
a transport stream packet instead of directly accessing the fifth element of a
std::array that maps only the first four bytes of the transport stream packet.

mythtv/libs/libmythtv/mpeg/tspacket.h

@@ -95,7 +95,7 @@ class MTV_PUBLIC TSHeader
     bool Scrambled(void) const { return bool(m_tsData[3]&0x80); }
     bool HasAdaptationField(void) const { return bool(m_tsData[3] & 0x20); }
     size_t AdaptationFieldSize(void) const
-    { return (HasAdaptationField() ? static_cast<size_t>(m_tsData[4]) : 0); }
+    { return (HasAdaptationField() ? static_cast<size_t>(data()[4]) : 0); }
     bool HasPayload(void) const { return bool(m_tsData[3] & 0x10); }
 
     bool GetDiscontinuityIndicator(void) const