introduced constant ZSTD_frameIdSize

within zstd_internal.h
This is the size of magic number.

Avoids using `4` directly in source code, which is a bit less meaningful.

lib/common/zstd_internal.h

@@ -105,6 +105,8 @@ static const U32 repStartValue[ZSTD_REP_NUM] = { 1, 4, 8 };
 static const size_t ZSTD_fcs_fieldSize[4] = { 0, 2, 4, 8 };
 static const size_t ZSTD_did_fieldSize[4] = { 0, 1, 2, 4 };
 
+static const size_t ZSTD_frameIdSize = 4;  /* magic number */
+
 #define ZSTD_BLOCKHEADERSIZE 3   /* C standard doesn't allow `static const` variable to be init using another `static const` variable */
 static const size_t ZSTD_blockHeaderSize = ZSTD_BLOCKHEADERSIZE;
 typedef enum { bt_raw, bt_rle, bt_compressed, bt_reserved } blockType_e;