Clippy cleanup

src/client.rs

@@ -12,7 +12,7 @@ impl ClientContext {
     fn get_user_name() -> String {
         env::var("USER")
             .or_else(|_| env::var("USERNAME"))
-            .unwrap_or("default".to_string())
+            .unwrap_or_else(|_| "default".to_string())
     }
 
     /// Get the default folder used by the Media Driver to interact with clients

src/driver.rs

@@ -95,10 +95,10 @@ impl MediaDriver {
 
 impl Drop for MediaDriver {
     fn drop(&mut self) {
-        if self.c_driver != ptr::null_mut() {
+        if self.c_driver.is_null() {
             unsafe { aeron_op!(aeron_driver_close(self.c_driver)) }.unwrap();
         }
-        if self.c_context != ptr::null_mut() {
+        if self.c_context.is_null() {
             unsafe { aeron_op!(aeron_driver_context_close(self.c_context)) }.unwrap();
         }
     }