Bug 1552363 - Initialize FreeSpan in default constructor r=jonco

Differential Revision: https://phabricator.services.mozilla.com/D55879

--HG--
extra : moz-landing-system : lando

js/src/gc/GC-inl.h

@@ -108,7 +108,9 @@ class ArenaCellIter {
         arenaAddr(nullptr),
         thing(0),
         traceKind(JS::TraceKind::Null),
-        initialized(false) {}
+        initialized(false) {
+    span.initAsEmpty();
+  }
 
   explicit ArenaCellIter(Arena* arena) : initialized(false) { init(arena); }