Amiga: defensive NULL/bounds guards in menus and window creation

Guard the gd lookup in DoMenuScroll's GADGETUP/MOUSEMOVE branches
so a window with no GadgetID==1 does not deref NULL; match the
existing guards in the keyboard-scroll branches.  In the keyboard
selector and MENU_UNSELECT_ALL paths, only mutate items with
canselect set so a non-selectable header cannot have its str
stomped.  Clamp MENU_LAST_PAGE topidx to >= 0.  Make find_menu_item
return NULL on negative idx instead of the head item.  Guard the
PROMPTFIRST data[] shuffle behind cury > 0.

In amii_destroy_nhwindow's NHW_OVER branch use cw->win with a NULL
guard instead of dereferencing amii_wins[WIN_OVER]->win blindly.
Range-check the type argument to amii_create_nhwindow.  Fix the
*argv_in[1] precedence bug so the -L/-l flag does not deref NULL
when it is the last argument.  Wrap AllocAslRequest result in a
NULL check before AslRequestTags/FreeAslRequest.

Defensively bounds-check the idx argument to DispCol.  Replace
the -25937 signed-int literal in clipwin's PropInfo with the
equivalent UWORD value 39599.  Simplify amii_start_menu's free
loop; switch DoMenuScroll's inventory title and Count display to
Snprintf, and stop passing countString to pline as a format.

sys/amiga/clipwin.c

@@ -197,7 +197,7 @@ static struct Gadget ClipXSIZE = {
 
 static struct PropInfo ClipClipYSIZESInfo = {
     AUTOKNOB + FREEHORIZ, /* PropInfo flags */
-    -25937, -1,           /* horizontal and vertical pot values */
+    39599, -1,            /* horizontal and vertical pot values */
     10922, -1,            /* horizontal and vertical body values */
 };