BUG: fixed SAS7BDATReader._get_properties
this fixes a crasher I had in one of my files using SAS7BDATReader.

cc: @kshedden 

self._path_or_buf.read is an obvious mistake

also added + total_align to _os_name_offset and _os_make_offset as they are
present in the original code from Jared Hobbs:
https://bitbucket.org/jaredhobbs/sas7bdat/src/da1faa90d0b15c2c97a2a8eb86c91c58081bdd86/sas7bdat.py?fileviewer=file-view-default#sas7bdat.py-1450

can you add a test that hits this path
 See #12659 test file. It fails too.
https://github.com/pydata/pandas/blob/8857008178eb90b59ca67874507ef860b3faf88f/pandas/io/tests/sas/data/test17.sas7bdat
 Do you want me to include that test file in this PR even if the file is **not** readable even with this PR (due to #12659)?
 prob easiest to have a single PR with the test files.
 btw, this fixes issue #12647 
 I've uploaded a test file here: https://github.com/raderaj/pandas_SAS_snippets/blob/master/productsales.sas7bdat 
 closed by #12656 
