I1620: Fixed bug in select index A (COVERITY)

If the CPU is a model 1, then the value of idxb is irrelevant. For
tidiness, it should always be 0, but on a model 1, idxe (index
enable) is always 0, so idxb is not looked at. Thus,

- on a model 1, idxb is set to 0, which is harmless and probably a good
   thing to do.
- on a model 2, idxe is set to 1, and idxb is set to 0, which is the correct
   behavior.

Note that case 0 doesn't need to check for model 2, because clearing
idxe and idxb yields the correct settings on a model 1: indexing is off.

I1620/i1620_cpu.c

@@ -1,6 +1,6 @@
 /* i1620_cpu.c: IBM 1620 CPU simulator
 
-   Copyright (c) 2002-2017, Robert M. Supnik
+   Copyright (c) 2002-2018, Robert M. Supnik
 
    Permission is hereby granted, free of charge, to any person obtaining a
    copy of this software and associated documentation files (the "Software"),
@@ -26,6 +26,7 @@
    This CPU module incorporates code and comments from the 1620 simulator by
    Geoff Kuenning, with his permission.
 
+   05-Jun-18    RMS     Fixed bug in select index A (COVERITY)
    23-Jun-17    RMS     BS should not enable indexing unless configured
    15-Jun-17    RMS     Added more information to IO in progress message
    26-May-17    RMS     Added deferred IO mode for slow devices
@@ -975,7 +976,7 @@ while (reason == SCPE_OK) {                             /* loop until halted */
             break;
         case 1:
             if ((cpu_unit.flags & IF_IDX) != 0)         /* indexing present? */
-                idxe = 1; idxb = 0;                     /* index band A */
+                idxe = 1, idxb = 0;                     /* index band A */
             break;
         case 2:
             if ((cpu_unit.flags & IF_IDX) != 0)         /* indexing present? */