Read only the mpeg header when reading packets.

When trying to convince FFmpeg to read only the header, this was allowed
to get longer.  But we only really need the actual header here, and that
can prevent FFmpeg mis-parsing later packets.

Core/HW/MediaEngine.cpp

@@ -396,8 +396,12 @@ int MediaEngine::addStreamData(const u8 *buffer, int addSize) {
 #ifdef USE_FFMPEG
 		if (!m_pFormatCtx && m_pdata->getQueueSize() >= 2048) {
 			m_mpegheaderSize = m_pdata->get_front(m_mpegheader, sizeof(m_mpegheader));
-			m_pdata->pop_front(0, m_mpegheaderSize);
-			openContext();
+			int streamOffset = (int)(*(s32_be *)(m_mpegheader + 8));
+			if (streamOffset <= m_mpegheaderSize) {
+				m_mpegheaderSize = streamOffset;
+				m_pdata->pop_front(0, m_mpegheaderSize);
+				openContext();
+			}
 		}
 #endif // USE_FFMPEG