prores: Extend the padding check to 16bit

Some files produced by the official encoder have up to 16bit of
padding instead of the expected padding to the byte.

Use a self-explanatory macro instead of a simple number.

CC: libav-stable@libav.org
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavcodec/proresdec.c

@@ -365,6 +365,7 @@ static inline void decode_dc_coeffs(GetBitContext *gb, int16_t *out,
     }
 }
 
+#define MAX_PADDING 16
 
 /**
  * Decode AC coefficients for all blocks in a slice.
@@ -389,7 +390,7 @@ static inline int decode_ac_coeffs(GetBitContext *gb, int16_t *out,
         lev_cb_index = ff_prores_lev_to_cb_index[FFMIN(level, 9)];
 
         bits_left = get_bits_left(gb);
-        if (bits_left <= 0 || (bits_left <= 8 && !show_bits(gb, bits_left)))
+        if (bits_left <= 0 || (bits_left <= MAX_PADDING && !show_bits(gb, bits_left)))
             return 0;
 
         run = decode_vlc_codeword(gb, ff_prores_ac_codebook[run_cb_index]);
@@ -397,7 +398,7 @@ static inline int decode_ac_coeffs(GetBitContext *gb, int16_t *out,
             return AVERROR_INVALIDDATA;
 
         bits_left = get_bits_left(gb);
-        if (bits_left <= 0 || (bits_left <= 8 && !show_bits(gb, bits_left)))
+        if (bits_left <= 0 || (bits_left <= MAX_PADDING && !show_bits(gb, bits_left)))
             return AVERROR_INVALIDDATA;
 
         level = decode_vlc_codeword(gb, ff_prores_ac_codebook[lev_cb_index]) + 1;