mpegaudiodec: fix seeking.

The safe bitstream reader does not allow using skip_bits_long() to seek to a
point before the start of the buffer, which was needed by the mp3 decoder.
This change instead calculates the start point of the first valid granule and
skips to that position.
Signed-off-by: Justin Ruggles <justin.ruggles@gmail.com>

libavcodec/mpegaudiodec.c

@@ -1551,25 +1551,27 @@ static int mp_decode_layer3(MPADecodeContext *s)
 #if !UNCHECKED_BITSTREAM_READER
         s->gb.size_in_bits_plus8 += EXTRABYTES * 8;
 #endif
-        skip_bits_long(&s->gb, 8*(s->last_buf_size - main_data_begin));
-    }
-
-    for (gr = 0; gr < nb_granules; gr++) {
-        for (ch = 0; ch < s->nb_channels; ch++) {
-            g = &s->granules[ch][gr];
-            if (get_bits_count(&s->gb) < 0) {
-                av_log(s->avctx, AV_LOG_DEBUG, "mdb:%d, lastbuf:%d skipping granule %d\n",
-                       main_data_begin, s->last_buf_size, gr);
-                skip_bits_long(&s->gb, g->part2_3_length);
+        s->last_buf_size <<= 3;
+        for (gr = 0, ch = 0; gr < nb_granules && (s->last_buf_size >> 3) < main_data_begin; gr++, ch = 0) {
+            for (; ch < s->nb_channels && (s->last_buf_size >> 3) < main_data_begin; ch++) {
+                g = &s->granules[ch][gr];
+                s->last_buf_size += g->part2_3_length;
                 memset(g->sb_hybrid, 0, sizeof(g->sb_hybrid));
-                if (get_bits_count(&s->gb) >= s->gb.size_in_bits && s->in_gb.buffer) {
-                    skip_bits_long(&s->in_gb, get_bits_count(&s->gb) - s->gb.size_in_bits);
-                    s->gb           = s->in_gb;
-                    s->in_gb.buffer = NULL;
-                }
-                continue;
             }
+        }
+        skip_bits_long(&s->gb, s->last_buf_size - 8 * main_data_begin);
+        if (get_bits_count(&s->gb) >= s->gb.size_in_bits && s->in_gb.buffer) {
+            skip_bits_long(&s->in_gb, get_bits_count(&s->gb) - s->gb.size_in_bits);
+            s->gb           = s->in_gb;
+            s->in_gb.buffer = NULL;
+        }
+    } else {
+        gr = ch = 0;
+    }
 
+    for (; gr < nb_granules; gr++, ch = 0) {
+        for (; ch < s->nb_channels; ch++) {
+            g = &s->granules[ch][gr];
             bits_pos = get_bits_count(&s->gb);
 
             if (!s->lsf) {