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: I1128903e8f429d8903b5282d7c1bd4aa0be00979