ID32: Fix DP unit busy test

The problem turned out to be a call to sim_is_active() from id_dp.c with
an invalid pointer. This surprised me some since this isn’t a module that
changed significantly.

In normal disk operation the driver accesses the disk address (sometimes
called file address in the hardware docs) before accessing the controller
address. The simh disk emulator (dp.c) sets a current disk variable,
dp_svun, to the current disk address when a disk is accessed. It gets
set to 0 initially and under some other circumstances.

The address of passed to sim_is_active() is calculated, in part, by
subtracting the controller address from the disk address. This results
in a bigish negative offset from the beginning of an array if dp_svun
is 0.

The diagnostic sends a disarm-disable (x’c0’) command to every device
address, from 0 to x’3ff’. This hits the controller before the disk,
exercising the error.

This same thing happened in v38-1, but sim_is_active() did not
dereference the pointer passed to it, but scanned an array for that
pointer value. The current version does dereference the pointer.

My crude fix, which does not make me happy, is to replace the call
sim_is_active(uptr) with (dp_svun && sim_is_active(uptr)). This does
give behavior identical to v38-1 and allows me to proceed.

Interdata/id_dp.c

@@ -297,7 +297,7 @@ switch (op) {                                           /* case IO op */
             }
         u = (dp_svun - dp_dib.dno - o_DP0) / o_DP0;     /* get unit */
         uptr = dp_dev.units + u;                        /* ignore if busy */
-        if (!(dp_sta & STC_IDL) || sim_is_active (uptr))
+        if (!(dp_sta & STC_IDL) || (dp_svun && sim_is_active (uptr)))
             break;
         dp_cmd = f;                                     /* save cmd */
         if (dp_cmd == CMC_WR)                           /* write: bsy=0 else */