avcodec/lpc: Avoid floating point division by 0

Fixes: Ticket7996
Fixes: CVE-2020-20445

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/lpc.c

@@ -189,7 +189,7 @@ double ff_lpc_calc_ref_coefs_f(LPCContext *s, const float *samples, int len,
     compute_ref_coefs(autoc, order, ref, error);
     for (i = 0; i < order; i++)
         avg_err = (avg_err + error[i])/2.0f;
-    return signal/avg_err;
+    return avg_err ? signal/avg_err : NAN;
 }
 
 /**

libavcodec/lpc.h

@@ -143,7 +143,7 @@ static inline void compute_ref_coefs(const LPC_TYPE *autoc, int max_order,
         gen0[i] = gen1[i] = autoc[i + 1];
 
     err    = autoc[0];
-    ref[0] = -gen1[0] / err;
+    ref[0] = -gen1[0] / ((USE_FIXED || err) ? err : 1);
     err   +=  gen1[0] * ref[0];
     if (error)
         error[0] = err;
@@ -152,7 +152,7 @@ static inline void compute_ref_coefs(const LPC_TYPE *autoc, int max_order,
             gen1[j] = gen1[j + 1] + ref[i - 1] * gen0[j];
             gen0[j] = gen1[j + 1] * ref[i - 1] + gen0[j];
         }
-        ref[i] = -gen1[0] / err;
+        ref[i] = -gen1[0] / ((USE_FIXED || err) ? err : 1);
         err   +=  gen1[0] * ref[i];
         if (error)
             error[i] = err;