Make WeakHandleBase::mImpl as unique_ptr + nullcheck for Reset and comparision

Make WeakHandleBase::mImpl as unique_ptr + nullcheck for Reset and comparision

Until now, WeakHandleBase::Reset() function and == operator
don't check mImpl was nullptr.

But it could be nullptr when we use move operations.

So we should check nullptr before call mImpl->Reset() and operator==.

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Also, to avoid this kind of leak, let we use std::unique_ptr instead of raw pointer.

Change-Id: Icd0b603b825d6a1f5eb7697466da98af05540b2b
Signed-off-by: Eunki, Hong <eunkiki.hong@samsung.com>