Fix typo in return value

VENDOR_TAG_NAME_ERR, which is NULL, was being returend by a function
that returns int.  When NULL is defined as 0 that compiles, but the
C++ spec also allows NULL to be std::nullptr, which cannot be implicitly
cast to int.  Use VENDOR_TAG_TYPE_ERR instead.

Fixes compiling against musl libc, which uses std::nullptr for NULL.

Bug: 190084016
Test: m USE_HOST_MUSL=true host-native
Change-Id: Ib14a2ada791ef59dc37e0a0d1d9e401ce86d7d6f