Fix calling destructor on uninitialized dynamic library crash.

A test case was also created for this situation to prevent the problem
occuring again.

A similar problem was also fixed for the symbol method.

There was some minor code cleanup.
auto merge of #8219 : sstewartgallus/rust/fix_dynamic_lib, r=graydon

A test case was also created for this situation to prevent the problem
occuring again.

A similar problem was also fixed for the symbol method.

There was some minor code cleanup.

I am unsatisfied with using /dev/null as an invalid dynamic library. It is not cross platform.