drv/acia: fix broken buffer bounds

How did this ever work?

drv/acia.fs

@@ -36,11 +36,15 @@ ACIA_MEM: Address in memory that can be used variables shared
 ;
 
 : KEY
-    ( As long as R> == W>-1, it means that buffer is empty )
-    BEGIN ACIAR> @ 1 + ACIAW> @ = NOT UNTIL
-
     ( inc then fetch )
-    1 ACIAR> +!
+    ACIAR> @ 1 + DUP ACIA) @ = IF
+        DROP ACIA( @
+    THEN
+
+    ( As long as R> == W>-1, it means that buffer is empty )
+    BEGIN DUP ACIAW> @ = NOT UNTIL
+
+    ACIAR> !
     ACIAR> @ C@
 ;
 

drv/acia.z80

@@ -39,11 +39,11 @@ ACIA_IO
     ( advance W> )
     HL INCss,
     ACIAW> LD(nn)HL,
-    DE ACIA) @ LDdd(nn),
+    DE ACIA) LDdd(nn),
     DE SUBHLss,
     JRNZ, L4 FWR ( skip )
     ( end of buffer reached )
-    ACIA( @ LDHL(nn),
+    ACIA( LDHL(nn),
     ACIAW> LD(nn)HL,
 L4 FSET ( skip )
 L3 FSET L2 FSET ( end )