vorbisdec: support freeing partially allocated contexts.

Fixes null pointer derefernces
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/vorbisdec.c

@@ -195,6 +195,7 @@ static void vorbis_free(vorbis_context *vc)
     av_freep(&vc->channel_residues);
     av_freep(&vc->saved);
 
+    if (vc->residues)
         for (i = 0; i < vc->residue_count; i++)
             av_free(vc->residues[i].classifs);
     av_freep(&vc->residues);
@@ -203,12 +204,14 @@ static void vorbis_free(vorbis_context *vc)
     ff_mdct_end(&vc->mdct[0]);
     ff_mdct_end(&vc->mdct[1]);
 
+    if (vc->codebooks)
         for (i = 0; i < vc->codebook_count; ++i) {
             av_free(vc->codebooks[i].codevectors);
             ff_free_vlc(&vc->codebooks[i].vlc);
         }
     av_freep(&vc->codebooks);
 
+    if (vc->floors)
         for (i = 0; i < vc->floor_count; ++i) {
             if (vc->floors[i].floor_type == 0) {
                 av_free(vc->floors[i].data.t0.map[0]);
@@ -221,6 +224,7 @@ static void vorbis_free(vorbis_context *vc)
         }
     av_freep(&vc->floors);
 
+    if (vc->mappings)
         for (i = 0; i < vc->mapping_count; ++i) {
             av_free(vc->mappings[i].magnitude);
             av_free(vc->mappings[i].angle);