Do not check zip-entry validity on create_namespace

This operation is the most expensive one and since it is
unlikely that the zip-entry name is invalid (given that it
is constructed by the platform) - it can removed.

The worst case scenario is dlopen() of non-existing libraries
taking more time. And this use-case is not on the critical path.

Bug: http://b/28801010
Change-Id: I10a6b0bf342404ab72f0f5102ebf19f6c06ee6bf
(cherry picked from commit a5c1c8e8)