libipt, section: fix truncation bug

Creating a file-based section of size UINT64_MAX, i.e. 'until end of file',
with a non-zero offset fails.  Adding this huge size to the offset causes
a wrap-around which results in a subsequent check to fail.

Adjust the size earlier to avoid the wrap-around.

Change-Id: I4496d6e2ce2b9bf44cfd54bd9318cb2acf93410f
Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>