build: gcc 5 does no longer implicitly converts nullptr to false,

see:
https://gcc.gnu.org/gcc-5/porting_to.html

Refs #73855

Change-Id: I26d8f987c46fd3c93b36cac061542de3ef1589d4

src/ring_api.cpp

@@ -66,7 +66,7 @@ init(enum InitFlag flags) noexcept
         // current implementation use static variable
         return &ring::Manager::instance() != nullptr;
     } catch (...) {
-        return nullptr;
+        return false;
     }
 }