MAINT-3703 Suspected thread race crasher in fmodex library

Two problems found in DLL involving threads.  First, DllMain
was reinitializing a critical section for all entry reasons
(process attach, detach and thread attach, detach).  Should
only be done on process attach.  Second, static container
object was being modified and accessed without serialization.
Added some double-check locking to the initialization path
to reduce the total number of serialization calls made while
making the code thread safe.