Use base::SafeRound instead of std::round.

Previous assertion violations because of NaN from std::round were
in video streaming, see commits 27d58ba07b, 8f5830d520.

Now the crashes happened in the ConvertScale() call from a background
thread when preparing an image from clipboard for sending to a chat.

Telegram/SourceFiles/mtproto/session_private.cpp

@@ -544,7 +544,9 @@ MTPVector<MTPJSONObjectValue> SessionPrivate::prepareInitParams() {
 		sliced -= 24 * 3600;
 	}
 	const auto sign = (sliced < 0) ? -1 : 1;
-	const auto rounded = std::round(std::abs(sliced) / 900.) * 900 * sign;
+	const auto rounded = base::SafeRound(std::abs(sliced) / 900.)
+		* 900
+		* sign;
 	return MTP_vector<MTPJSONObjectValue>(
 		1,
 		MTP_jsonObjectValue(