src: mark AsyncWrap destructor as virtual
src: mark AsyncWrap destructor as virtual

Make AsyncWrap::~AsyncWrap() virtual, otherwise it is unsafe to delete
a derived class through a pointer to AsyncWrap.  Fortunately, we don't
do that anywhere right now; this commit is merely a preemptive bug fix.

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node