GH-725: fix logic for finding a lambda's usage of outer IDs

testing/btest/language/lambda-nested-copy.zeek

@@ -0,0 +1,19 @@
+# @TEST-EXEC: zeek -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+
+local outer = 100;
+
+local lambda = function()
+	{
+	local inner = function(a: count, b: count, c: count, d: count, e: count, f: count)
+		{
+		print outer + f;
+		};
+
+	inner(1, 2, 3, 4, 5, 6);
+	};
+
+lambda();
+
+local copyLambda = copy(copy(copy(lambda)));
+copyLambda();