Fix issue with a safepoint happening before language.getCurrentThread() is set in non-root Fibers

* Unfortunately this cannot fix the case of using --fiber-leave-context=false.

Author: eregon

src/main/java/org/truffleruby/language/SafepointAction.java

@@ -57,9 +57,9 @@ protected final void perform(Access access) {
                             Thread.currentThread());
         }
 
-        final RubyLanguage language = RubyLanguage.getCurrentLanguage();
         final RubyContext context = RubyLanguage.getCurrentContext();
-        final RubyThread rubyThread = language.getCurrentThread();
+        // Not using language.getCurrentThread() here because it might not be set yet, see fiberMain()
+        final RubyThread rubyThread = context.getThreadManager().getRubyThreadForJavaThread(access.getThread());
         if (filter.test(context, rubyThread, this)) {
             run(rubyThread, access.getLocation());
         }