avcodec/amrwbdec: Fix division by 0 in find_hb_gain()

This restructures the code slightly toward D_UTIL_dec_synthesis()

Fixes: 7420/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AMRWB_fuzzer-6577305112543232

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

libavcodec/amrwbdec.c

@@ -862,15 +862,20 @@ static float find_hb_gain(AMRWBContext *ctx, const float *synth,
 {
     int wsp = (vad > 0);
     float tilt;
+    float tmp;
 
     if (ctx->fr_cur_mode == MODE_23k85)
         return qua_hb_gain[hb_idx] * (1.0f / (1 << 14));
 
-    tilt = ctx->celpm_ctx.dot_productf(synth, synth + 1, AMRWB_SFR_SIZE - 1) /
-           ctx->celpm_ctx.dot_productf(synth, synth, AMRWB_SFR_SIZE);
+    tmp = ctx->celpm_ctx.dot_productf(synth, synth + 1, AMRWB_SFR_SIZE - 1);
+
+    if (tmp > 0) {
+        tilt = tmp / ctx->celpm_ctx.dot_productf(synth, synth, AMRWB_SFR_SIZE);
+    } else
+        tilt = 0;
 
     /* return gain bounded by [0.1, 1.0] */
-    return av_clipf((1.0 - FFMAX(0.0, tilt)) * (1.25 - 0.25 * wsp), 0.1, 1.0);
+    return av_clipf((1.0 - tilt) * (1.25 - 0.25 * wsp), 0.1, 1.0);
 }
 
 /**