Don't perform field inverse/squaring during affine conversion if point is already in affine.

tests/serialization.rs

@@ -35,6 +35,7 @@ fn group_serialization_and_deserialization() {
         a = a * b;
 
         assert!(reserialize(a) == a);
+        assert!(reserialize(reserialize(a)) == a);
     }
 
     let mut a = G2::one();
@@ -42,6 +43,7 @@ fn group_serialization_and_deserialization() {
         a = a * b;
 
         assert!(reserialize(a) == a);
+        assert!(reserialize(reserialize(a)) == a);
     }
 }