initialize count_ from starting point in MyStackWalker(MyStackWalker*)

When we're initializing a stack walker based on an existing instance,
we should initialize all the fields from that instance, including
count_.

src/compile.cpp

@@ -532,7 +532,7 @@ class MyStackWalker: public Processor::StackWalker {
     method_(w->method_),
     target(w->target),
     continuation(w->continuation),
-    count_(0),
+    count_(w->count_),
     protector(this)
   { }