Fix a couple of warnings

- Conversion warning on clang: 'long' to 'size_t'
- Unused variable warning on MSVC
diff --git a/example/issues.cpp b/example/issues.cpp
index dfe20ff..d75da1c 100644
--- a/example/issues.cpp
+++ b/example/issues.cpp
@@ -135,7 +135,7 @@
     try {
         py::class_<Placeholder>(m2, "Placeholder");
         throw std::logic_error("Expected an exception!");
-    } catch (std::runtime_error &e) {
+    } catch (std::runtime_error &) {
         /* All good */
     }
 }