event/PollGroupWinSelect: move IsEmpty() check to GetPtr()

src/event/PollGroupWinSelect.cxx

@@ -142,9 +142,9 @@ PollGroupWinSelect::ReadEvents(PollResultGeneric &result,
 	}
 
 	int ret = select(0,
-			 read_set.IsEmpty() ? nullptr : read_set.GetPtr(),
-			 write_set.IsEmpty() ? nullptr : write_set.GetPtr(),
-			 except_set.IsEmpty() ? nullptr : except_set.GetPtr(),
+			 read_set.GetPtr(),
+			 write_set.GetPtr(),
+			 except_set.GetPtr(),
 			 timeout_ms < 0 ? nullptr : &tv);
 
 	if (ret == 0 || ret == SOCKET_ERROR)

src/event/PollGroupWinSelect.hxx

@@ -52,7 +52,7 @@ public:
 	}
 
 	fd_set *GetPtr() noexcept {
-		return &set;
+		return IsEmpty() ? nullptr : &set;
 	}
 
 	size_t Size() const noexcept {