Return 0 instead of NULL for jlong

musl libc defines NULL as nullptr, which is explicitly allowed by
C++11.  nullptr_t cannot be implicitly cast to an integral type.
Return 0 instead.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: Id83c1994a95cb105785c8ad902d2fe0589cb188d
2 files changed