vlc_shared_data_ptr: make operator bool() explicit

Like unique_ptr, make the operator bool() explicit, so that it is only
called implicitly when the value may be "contextually converted to bool"
(if, while, etc.).
Suggested-by: Filip Roséen <filip@atch.se>
Signed-off-by: Thomas Guillem <thomas@gllm.fr>

include/vlc_cxx_helpers.hpp

@@ -207,7 +207,7 @@ public:
         return !(*this == other);
     }
 
-    operator bool() const
+    explicit operator bool() const
     {
         return ptr;
     }