HEVC/h.265 ES reader: clear parser's buffer after reading the headers

After reading the headers, the file is read again from the
start. Therefore no data must be left in the parser's internal
buffers. That was the intention behind calling `flush()` - but that
isn't actually what flush does. Instead it tries to use the remaining
data in the buffers as a frame.

This is not only wrong, it can also fail and cause an exception to be
thrown. That in turn causes a failure when reading the headers — both
identification and muxing abort at that point.

This is the HEVC analog for what was fixed for AVC in #2325.

NEWS.md

@@ -8,6 +8,9 @@
 * mkvmerge: AVC/h.264: fixed file identification failing for certain
   elementary streams due to internal buffers not being cleared properly. Fixes
   #2325.
+* mkvmerge: HEVC/h.265: fixed file identification failing for certain
+  elementary streams due to internal buffers not being cleared properly. This
+  is the HEVC analog to what was fixed for AVC in #2325.
 
 
 # Version 24.0.0 "Beyond The Pale" 2018-06-10

src/common/hevc_es_parser.cpp

@@ -192,6 +192,13 @@ es_parser_c::flush() {
   cleanup();
 }
 
+void
+es_parser_c::clear() {
+  m_unparsed_buffer.reset();
+  m_have_incomplete_frame = false;
+  m_parsed_position       = 0;
+}
+
 void
 es_parser_c::add_timestamp(int64_t timestamp) {
   m_provided_timestamps.emplace_back(timestamp, m_stream_position);

src/common/hevc_es_parser.h

@@ -119,6 +119,7 @@ public:
   }
 
   void flush();
+  void clear();
 
   bool frame_available() {
     return !m_frames_out.empty();

src/input/r_hevc.cpp

@@ -89,14 +89,13 @@ hevc_es_reader_c::read_headers() {
         break;
     }
 
-    if (parser.headers_parsed())
-      parser.flush();
-
     m_width  = parser.get_width();
     m_height = parser.get_height();
 
     m_in->setFilePointer(0, seek_beginning);
 
+    parser.clear();
+
   } catch (...) {
     throw mtx::input::open_x();
   }