Fix memset call in xdl/data_io/packer/pack_feature

[disconnect3d]

The void *memset(void *s, int c, size_t n) function takes the value to fill in its 2nd argument and the number of byte to fill in in 3rd.

This commit fixes an invalid use of memset which had the two arguments swapped and it did not set any memory at all in the end due to that.

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