src,http: fix uncaughtException miss in http

In AsyncWrap::MakeCallback always return empty handle if there is an
error. In the future this should change to return a v8::MaybeLocal, but
that major change will have to wait for v6.x, and these changes are
meant to be backported to v4.x.

The HTTParser call to AsyncWrap::MakeCallback failed because it expected
a thrown call to return an empty handle.

In node::MakeCallback return an empty handle if the call is
in_makecallback(), otherwise return v8::Undefined() as usual to preserve
backwards compatibility.

Fixes: https://github.com/nodejs/node/issues/5555
PR-URL: https://github.com/nodejs/node/pull/5591


Reviewed-By: Julien Gilli <jgilli@nodejs.org>