libipt: clear insn in pt_insn_next()

In pt_insn_next() we clear a number of bit-fields that are not guaranteed
to be set on all paths.

Simply clear the entire insn object, instead.

Change-Id: Ib8e8be86ba2e4a2138d33478961d46b8b38bc8b8
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
libipt: rearrange functions in pt_insn.c

Group instruction decoding and analyzing functions together.  Move them near
the top of the file.

Change-Id: I6c6421a9936ae80d3121a0ec6c766f4e03febf7e
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>