Bug 957: Assert that the ECMAScript interpreter is not running.

Add ecmascript_interpreter.backend_nesting, increment it when
beginning to evaluate an expression, and decrement it when evaluation
finishes.  Then assert that it is zero in ecmascript_put_interpreter.
This detects bug 957 and similar ones before they corrupt memory.

src/ecmascript/ecmascript.h

@@ -18,6 +18,12 @@ struct view_state;
 struct ecmascript_interpreter {
 	struct view_state *vs;
 	void *backend_data;
+
+	/* Nesting level of calls to backend functions.  When this is
+	 * nonzero, there are references to backend_data in the C
+	 * stack, so it is not safe to free the data yet.  */
+	int backend_nesting;
+
 	/* Used by document.write() */
 	struct string *ret;