Returns the table entry denoted by *idx* from header table of
*deflater*.  The *idx* is 1-based, and idx=1 returns first entry of
static table.  idx=62 returns first entry of dynamic table if it
exists.  Specifying idx=0 is error, and this function returns NULL.
If *idx* is strictly greater than the number of entries the tables
contain, this function returns NULL.