avcodec/vorbisdec: Implement vr->classifications = 1

It appears no valid file uses this, so this is not testable with
a valid file.

Fixes: assertion failure
Fixes: 16187/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_VORBIS_fuzzer-5638880618872832

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 5a5f12e3b3)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vorbisdec.c

@@ -1361,8 +1361,12 @@ static av_always_inline int setup_classifs(vorbis_context *vc,
                 return AVERROR_INVALIDDATA;
             }
 
-            av_assert0(vr->classifications > 1); //needed for inverse[]
-
+            if (vr->classifications == 1) {
+                for (i = partition_count + c_p_c - 1; i >= partition_count; i--) {
+                    if (i < ptns_to_read)
+                        vr->classifs[p + i] = 0;
+                }
+            } else {
             for (i = partition_count + c_p_c - 1; i >= partition_count; i--) {
                 temp2 = (((uint64_t)temp) * inverse_class) >> 32;
 
@@ -1370,6 +1374,7 @@ static av_always_inline int setup_classifs(vorbis_context *vc,
                     vr->classifs[p + i] = temp - temp2 * vr->classifications;
                 temp = temp2;
             }
+            }
         }
         p += ptns_to_read;
     }