Fix potential linear alloc memory leak

Previously, if we created a linear alloc for a class loader but
never created the class table, the linear alloc would never get
freed since it would have no corresponding ClassLoaderData.

Fixes valgrind-test-art-host-gtest-oat_test

Bug: 27384882
Bug: 22858531

(cherry picked from commit 5b83050a)

Change-Id: I71b650eac4e33212a7f03c43141db99e635a19ad