H264Parser: Fix Coverity ID 700913 Uninitialized scalar field

In H264Parser::H264Parser(): Make sure pkt_offset is initialized in Reset() which
is called by the constructor to initialize the parser.

mythtv/libs/libmythtv/mpeg/H264Parser.cpp

@@ -148,7 +148,7 @@ void H264Parser::Reset(void)
     timeScale = 0;
     fixedRate = 0;
 
-    AU_offset = frame_start_offset = keyframe_start_offset = 0;
+    pkt_offset = AU_offset = frame_start_offset = keyframe_start_offset = 0;
     on_frame = on_key_frame = false;
 
     resetRBSP();