Fix: Don't allocate in Fiber.unsafe_each and Thread.unsafe_each (#…

…14635)

src/crystal/system/thread.cr

@@ -54,7 +54,9 @@ class Thread
   getter name : String?
 
   def self.unsafe_each(&)
-    threads.unsafe_each { |thread| yield thread }
+    # nothing to iterate when @@threads is nil + don't lazily allocate in a
+    # method called from a GC collection callback!
+    @@threads.try(&.unsafe_each { |thread| yield thread })
   end
 
   # Creates and starts a new system thread.

src/fiber.cr

@@ -73,7 +73,9 @@ class Fiber
 
   # :nodoc:
   def self.unsafe_each(&)
-    fibers.unsafe_each { |fiber| yield fiber }
+    # nothing to iterate when @@fibers is nil + don't lazily allocate in a
+    # method called from a GC collection callback!
+    @@fibers.try(&.unsafe_each { |fiber| yield fiber })
   end
 
   # Creates a new `Fiber` instance.