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