rtpdec_jpeg: Write the DHT section properly

Currently the size header of the generated DHT section is
incorrect, making the mjpeg decoder just skip it. Since the
written huffman tables are the default ones, this failure had
gone undetected.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpdec_jpeg.c

@@ -76,13 +76,12 @@ static void jpeg_free_context(PayloadContext *jpeg)
     av_free(jpeg);
 }
 
-static void jpeg_create_huffman_table(PutBitContext *p, int table_class,
-                                      int table_id, const uint8_t *bits_table,
-                                      const uint8_t *value_table)
+static int jpeg_create_huffman_table(PutBitContext *p, int table_class,
+                                     int table_id, const uint8_t *bits_table,
+                                     const uint8_t *value_table)
 {
     int i, n = 0;
 
-    put_bits(p, 8, 0);
     put_bits(p, 4, table_class);
     put_bits(p, 4, table_id);
 
@@ -94,12 +93,15 @@ static void jpeg_create_huffman_table(PutBitContext *p, int table_class,
     for (i = 0; i < n; i++) {
         put_bits(p, 8, value_table[i]);
     }
+    return n + 17;
 }
 
 static int jpeg_create_header(uint8_t *buf, int size, uint32_t type, uint32_t w,
                               uint32_t h, const uint8_t *qtable, int nb_qtable)
 {
     PutBitContext pbc;
+    uint8_t *dht_size_ptr;
+    int dht_size;
 
     init_put_bits(&pbc, buf, size);
 
@@ -142,15 +144,20 @@ static int jpeg_create_header(uint8_t *buf, int size, uint32_t type, uint32_t w,
 
     /* DHT */
     put_marker(&pbc, DHT);
-
-    jpeg_create_huffman_table(&pbc, 0, 0, avpriv_mjpeg_bits_dc_luminance,
-                              avpriv_mjpeg_val_dc);
-    jpeg_create_huffman_table(&pbc, 0, 1, avpriv_mjpeg_bits_dc_chrominance,
-                              avpriv_mjpeg_val_dc);
-    jpeg_create_huffman_table(&pbc, 1, 0, avpriv_mjpeg_bits_ac_luminance,
-                              avpriv_mjpeg_val_ac_luminance);
-    jpeg_create_huffman_table(&pbc, 1, 1, avpriv_mjpeg_bits_ac_chrominance,
-                              avpriv_mjpeg_val_ac_chrominance);
+    flush_put_bits(&pbc);
+    dht_size_ptr = put_bits_ptr(&pbc);
+    put_bits(&pbc, 16, 0);
+
+    dht_size  = 2;
+    dht_size += jpeg_create_huffman_table(&pbc, 0, 0,avpriv_mjpeg_bits_dc_luminance,
+                                          avpriv_mjpeg_val_dc);
+    dht_size += jpeg_create_huffman_table(&pbc, 0, 1, avpriv_mjpeg_bits_dc_chrominance,
+                                          avpriv_mjpeg_val_dc);
+    dht_size += jpeg_create_huffman_table(&pbc, 1, 0, avpriv_mjpeg_bits_ac_luminance,
+                                          avpriv_mjpeg_val_ac_luminance);
+    dht_size += jpeg_create_huffman_table(&pbc, 1, 1, avpriv_mjpeg_bits_ac_chrominance,
+                                          avpriv_mjpeg_val_ac_chrominance);
+    AV_WB16(dht_size_ptr, dht_size);
 
     /* SOF0 */
     put_marker(&pbc, SOF0);