Amiga: fix overview-window crashes

- MyAllocBitMap left bm->mflags uninitialized, so MyFreeBitMap took the
  wrong path between FreeRaster and FreeMem and intermittently corrupted
  exec's free list (Software Failure 0x81000005, DEADEND in FreeMem).
- The NHW_OVER window is BORDERLESS, so attaching WINDOWSIZING |
  WINDOWDRAG | WINDOWCLOSE created phantom gadgets that hit-test against
  unrelated input.  Pressing ESC while the overview was selected fired
  CLOSEWINDOW and destroyed the window underneath the running code,
  leading to wild-PC crashes.  Drop the gadget flags; SHIFT-HELP already
  toggles the overview cleanly via delayed_key_action.
- amii_destroy_nhwindow only reset WIN_MAP / WIN_STATUS / WIN_MESSAGE /
  WIN_INVEN; WIN_OVER and WIN_BASE kept pointing at freed slots, so any
  later 'WIN_X != WIN_ERR && amii_wins[WIN_X]->win' check dereferenced
  NULL.  Reset them too.

sys/amiga/winchar.c

@@ -189,6 +189,7 @@ MyAllocBitMap(int xsize, int ysize, int depth, long mflags)
     if (!bm)
         return (NULL);
 
+    bm->mflags = mflags;
     bm->xsize = xsize;
     bm->ysize = ysize;
     InitBitMap(&bm->bm, depth, xsize, ysize);

sys/amiga/winfuncs.c

@@ -227,6 +227,10 @@ amii_destroy_nhwindow(winid win) /* just hide */
         WIN_MESSAGE = WIN_ERR;
     else if (win == WIN_INVEN)
         WIN_INVEN = WIN_ERR;
+    else if (win == WIN_OVER)
+        WIN_OVER = WIN_ERR;
+    else if (win == WIN_BASE)
+        WIN_BASE = WIN_ERR;
 }
 
 struct FillParams {
@@ -416,8 +420,13 @@ amii_create_nhwindow(int type)
         if (nw->Width >= amiIDisplay->xpix - nw->LeftEdge)
             nw->Width = amiIDisplay->xpix - nw->LeftEdge;
     } else if (WINVERS_AMIV && type == NHW_OVER) {
-        nw->Flags |= WINDOWSIZING | WINDOWDRAG | WINDOWCLOSE;
+        /* No window-system gadgets: BORDERLESS means there is no border
-        nw->IDCMPFlags |= CLOSEWINDOW;
+         * to attach close/size/drag gadgets to.  On some Kickstarts the
+         * phantom gadgets hit-test against unrelated input (selecting
+         * the overview and pressing ESC has been observed to destroy
+         * the window and crash the game).  SHIFT-HELP toggles the
+         * overview cleanly via delayed_key_action; that is the
+         * supported close path. */
         /* Bring up window as half the width of the message window, and make
          * the message window change to one half the width...
          */