Replace obsolete safe bool idiom with explicit operator bool

Replace obsolete safe bool idiom with explicit operator bool

C++11 introduces 'explicit operator bool' to prevent unintended implicit
conversions to 'bool', thus making the trick with converting to a
pointer-to-member (a.k.a. "safe bool idiom") obsolete.

The explicit operator is more restrictive than 'safe bool', and it
helped uncover a bug in the test suite where object handles were
implicitly converted to bool before being sent to an std::ostream.

Change-Id: I8c9d33812bc022570f5c286d925914dba508185c