video/upd7220.cpp: return 0xff on FIFO underflows

* fix pc9821:skinpan intro hang

hash/pc98.xml

@@ -6767,8 +6767,10 @@ Never output any sound
 		<year>1993</year>
 		<publisher>ガイナックス (Gainax)</publisher>
 		<notes><![CDATA[
-Hangs during the intro (first dialogue)
+Hangs during the intro (first dialogue), [7220] FIFO underflows, worked around.
+Several [7220] invalid commands 0x05
 Draws Gainax logo with half height
+Title screen oddly keeps scrolling (verify)
 ]]></notes>
 		<info name="alt_title" value="バトルスキンパニック ９８２１" />
 		<info name="release" value="19930921" />

src/devices/video/upd7220.cpp

@@ -320,6 +320,12 @@ inline void upd7220_device::dequeue(uint8_t *data, int *flag)
 		if (m_fifo_ptr == -1)
 			m_sr &= ~UPD7220_SR_DATA_READY;
 	}
+	else
+	{
+		// TODO: underflow details
+		// pc9821:skinpan does SR checks over the wrong port during intro ...
+		*data = 0xff;
+	}
 }