src: fix addon loader regression

Fix a regression that was introduced in commit a38b9178 by removing the
bad check.  Also rearrange the addon loading logic to ensure that the
list of pending addons remains in a consistent state when the shared
object fails to load; in particular, when an addon self-registers first,
then hits a dynamic linker error in a later constructor.

Fixes the following asserting when loading a .node shared object:

    node: ../src/node.cc:1944: void node::node_module_register(void*):
    Assertion `(modpending) != (nullptr)' failed.

Fixes strongloop/strongops#233.

R=@trevnorris