ac3enc: add some assertions

(cherry picked from commit 2d9a101a)

libavcodec/ac3enc.c

@@ -428,6 +428,7 @@ static void extract_exponents(AC3EncodeContext *s)
                         e = 24;
                         coef[i] = 0;
                     }
+                    av_assert2(e >= 0);
                 }
                 exp[i] = e;
             }
@@ -650,16 +651,19 @@ static void group_exponents(AC3EncodeContext *s)
                 exp1   = p[0];
                 p     += group_size;
                 delta0 = exp1 - exp0 + 2;
+                av_assert2(delta0 >= 0 && delta0 <= 4);
 
                 exp0   = exp1;
                 exp1   = p[0];
                 p     += group_size;
                 delta1 = exp1 - exp0 + 2;
+                av_assert2(delta1 >= 0 && delta1 <= 4);
 
                 exp0   = exp1;
                 exp1   = p[0];
                 p     += group_size;
                 delta2 = exp1 - exp0 + 2;
+                av_assert2(delta2 >= 0 && delta2 <= 4);
 
                 block->grouped_exp[ch][i] = ((delta0 * 5 + delta1) * 5) + delta2;
             }
@@ -947,6 +951,7 @@ static int cbr_bit_allocation(AC3EncodeContext *s)
     int snr_offset, snr_incr;
 
     bits_left = 8 * s->frame_size - (s->frame_bits + s->exponent_bits);
+    av_assert2(bits_left >= 0);
 
     snr_offset = s->coarse_snr_offset << 4;
 
@@ -1438,6 +1443,7 @@ static void output_frame_end(AC3EncodeContext *s)
     frame_size_58 = ((s->frame_size >> 2) + (s->frame_size >> 4)) << 1;
 
     /* pad the remainder of the frame with zeros */
+    av_assert2(s->frame_size * 8 - put_bits_count(&s->pb) >= 18);
     flush_put_bits(&s->pb);
     frame = s->pb.buf;
     pad_bytes = s->frame_size - (put_bits_ptr(&s->pb) - frame) - 2;