Ensure that cx->interpState is always accurate by maintaing a stack (490776, r=jorendorff).

js/src/jstracer.cpp

@@ -4312,6 +4312,7 @@ js_ExecuteTree(JSContext* cx, Fragment* f, uintN& inlineCallCount,
 
     JS_ASSERT(!tm->tracecx);
     tm->tracecx = cx;
+    state->prev = cx->interpState;
     cx->interpState = state;
 
     debug_only(fflush(NULL);)
@@ -4325,9 +4326,7 @@ js_ExecuteTree(JSContext* cx, Fragment* f, uintN& inlineCallCount,
 
     AUDIT(traceTriggered);
 
-#ifdef DEBUG
-    cx->interpState = NULL;
-#endif
+    cx->interpState = state->prev;
 
     JS_ASSERT(lr->exitType != LOOP_EXIT || !lr->calldepth);
     tm->tracecx = NULL;

js/src/jstracer.h

@@ -385,6 +385,7 @@ struct InterpState
 #ifdef EXECUTE_TREE_TIMER
     uint64         startTime;
 #endif
+    InterpState*   prev;
 
     /*
      * Used by _FAIL builtins; see jsbuiltins.h. The builtin sets the