avcodec/jpeg2000dec: Improve readability of SOP check

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/jpeg2000.h

@@ -58,6 +58,9 @@ enum Jpeg2000Markers {
     JPEG2000_EOC = 0xffd9, // end of codestream
 };
 
+#define JPEG2000_SOP_FIXED_BYTES 0xFF910004
+#define JPEG2000_SOP_BYTE_LENGTH 6
+
 enum Jpeg2000Quantsty { // quantization style
     JPEG2000_QSTY_NONE, // no quantization
     JPEG2000_QSTY_SI,   // scalar derived

libavcodec/jpeg2000dec.c

@@ -812,8 +812,8 @@ static int jpeg2000_decode_packet(Jpeg2000DecoderContext *s, Jpeg2000Tile *tile,
         }
     }
 
-    if (bytestream2_peek_be32(&s->g) == 0xFF910004)
-        bytestream2_skip(&s->g, 6);
+    if (bytestream2_peek_be32(&s->g) == JPEG2000_SOP_FIXED_BYTES)
+        bytestream2_skip(&s->g, JPEG2000_SOP_BYTE_LENGTH);
 
     if (!(ret = get_bits(s, 1))) {
         jpeg2000_flush(s);