ART: Fix ArenaAllocator invariant check in Realloc

It is indeed possible to see end == ptr_ under sanitization in the
case that the caller Reallocs a (null, 0) allocation. This happens,
for example, in the classlinker during linking of methods (empty
interface method list).

Bug: 31098551
Test: ART_ENABLE_ADDRESS_SANITIZER=true SANITIZE_HOST=address m test-art-host
Change-Id: Ie79f27c39e07cc3793c0766fa8ea233b488dfaf0