usnic: fix invalid rfstart initialization

endpoint_rfstart was being initialized from a value which was not yet
set.  Also ensure that rfstart is a valid index in the range
0..WINDOW_SIZE-1, since it is used as the index into endpoint_rcvd_segs,
which has WINDOW_SIZE elements.

Without this change there is significant risk of memory corruption or
segfaults, resulting in hangs or crashes, if malloc ever returns us a
value >=WINDOW_SIZE (4096).  Right now we seem to be getting lucky that
the malloc is returning zero-pages to us when we are allocating endpoint
structures (possibly because the freelist performs a single large
allocation for all endpoints).

Fixes Cisco bug CSCui88781.

Reviewed-by: rfaucett@cisco.com
Reviewed-by: jsquyres@cisco.com

cmr=v1.7.3:reviewer=jsquyres