library: standardize extents_free_all() logic, 3rd gen

library: standardize extents_free_all() logic, 3rd gen

As those 3rd generation newlib APIs evolved so too did
the extents_free_all() function. Most versions of this
function required the callers to first verify that the
extents anchor wasn't empty, which was poor etiquette.

This simple function should have been much more robust
and forgiving. With this commit, it fnally becomes so.

Signed-off-by: Jim Warner <james.warner@comcast.net>