mesa: revert some recent VBO buffer object refcounting changes

mesa: revert some recent VBO buffer object refcounting changes

Reverts part of commit d7ea9ddf5824556e47decac7ba200f37cf1e552f.
We were calling _mesa_reference_buffer_object() on some heap-allocated
memory that was uninitialized and could trigger an assertion.
We can actually go back to "looser" ref counting of the Null/default
buffer object in these cases.