avcodec/wmadec: Keep track of exponent initialization per channel

Fixes: division by 0
Fixes: 19123/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_WMAV2_fuzzer-5655493121146880

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wma.h

@@ -123,7 +123,7 @@ typedef struct WMACodecContext {
     uint8_t last_superframe[MAX_CODED_SUPERFRAME_SIZE + AV_INPUT_BUFFER_PADDING_SIZE]; /* padding added */
     int last_bitoffset;
     int last_superframe_len;
-    int exponents_initialized;
+    int exponents_initialized[MAX_CHANNELS];
     float noise_table[NOISE_TAB_SIZE];
     int noise_index;
     float noise_mult; /* XXX: suppress that and integrate it in the noise array */

libavcodec/wmadec.c

@@ -585,11 +585,14 @@ static int wma_decode_block(WMACodecContext *s)
                     decode_exp_lsp(s, ch);
                 }
                 s->exponents_bsize[ch] = bsize;
+                s->exponents_initialized[ch] = 1;
             }
         }
-        s->exponents_initialized = 1;
-    }else if (!s->exponents_initialized) {
-        return AVERROR_INVALIDDATA;
+    }
+
+    for (ch = 0; ch < s->avctx->channels; ch++) {
+        if (s->channel_coded[ch] && !s->exponents_initialized[ch])
+            return AVERROR_INVALIDDATA;
     }
 
     /* parse spectral coefficients : just RLE encoding */