Re-enable tests in CI (aws#113)

also fix crash

Author: graebm

builder.json

@@ -26,5 +26,8 @@
                 }
             }
         }
-    }
+    },
+    "test_steps": [
+        "test"
+    ]
 }

source/io/Bootstrap.cpp

@@ -33,7 +33,9 @@ namespace Aws
                     auto callbackData = static_cast<ClientBootstrapCallbackData *>(userData);
 
                     callbackData->ShutdownPromise.set_value();
-                    callbackData->ShutdownCallback();
+                    if (callbackData->ShutdownCallback) {
+                        callbackData->ShutdownCallback();
+                    }
 
                     delete callbackData;
                 }