Bug 19313 – Attaching external thread may result in segfault

Split from https://issues.dlang.org/show_bug.cgi?id=19288 as on of sub-issues Essence of the issue: ``` import core.sys.posix.pthread; import core.memory; extern(C) void* entry_point(void*) { // in real world case this may be called through `thread_attachThis`: GC.collect(); return null; } void main() { // ensure some garbage exists auto x = new int[1000]; pthread_t thread; auto status = pthread_create(&thread, null, &entry_point, null); assert(status == 0); pthread_join(thread, null); } ``` Same applies to other operating systems. Problems comes from the fact that GC implementation is not ready for `Thread.getThis()` to be `null` and will crash in several places if this happens. And because `thread_attachThis` has to call `new Thread` to allocate new context, collection may happen before thread is actually registered.

fixed by https://github.com/dlang/druntime/commit/42b4e0a9614ac794d4549ed5b2455fd0f805e123#diff-6615611ba51118215a271e3500c61122