Apply same vetting to SetCaretPosition window handle as non cut & paste Wimp

Detail:
  Summary of how non-CnP Wimp treats the window handle for Wimp_SetCaretPosition
  R0 = valid window handle
    or invalid window handle (mapped to -1)
    or any negative number or 0 (mapped to -1)

  With this change, the CnP Wimp does
  R0 = valid window handle
    or invalid window handle (throws error)
    or any negative number or 0 if pre RISC OS 5 (mapped to -1)
    or 0 for RISC OS 5 (mapped to -1)

  Noting the use of AcceptLoosePointer macro if built for a 26 bit target.

s/CnPCaret

@@ -29,7 +29,6 @@
 int_set_caret_position ROUT
         Push    "R0-R6,R11,R14"
 
-      [ CnP
         ; if we're called with R5=-1 then R4,R5 are calculated from R0-R3
         ; however, R4 is undefined on entry so could be anything.
         ; Internally we'd like to be able to use the call to put a ghost caret
@@ -44,7 +43,6 @@ int_set_caret_position ROUT
 00
         CMP     R5,#-2
         MOVEQ   R5,#-1
-      ]
 
         ; validate existing caret blocks
         LDR     handle,caretdata
@@ -98,7 +96,8 @@ int_set_caret_position ROUT
         BL      iconautoscroll_stop
         Pull    "R0,x0,y0,x1,y1"
 00
-        CMP     R0,#-1
+        AcceptLoosePointer_NegOrZero R0,nullptr
+        CMP     R0,R0,ASR #31
         BEQ     %FT07                   ; we know this one's not valid
         MOV     handle,R0
         BL      checkhandle             ; valid window handle?
@@ -109,7 +108,7 @@ int_set_caret_position ROUT
         LDR     R14,taskidentifier1     ; 'TASK'
         CMP     R2,R14                  ; is R2 'TASK'
         MOVNE   R4,#0                   ; if we're called with TASK then R4 has correct flags.  If not, we need
-        ; to make sure we have some valid flags
+                                        ; to make sure we have some valid flags
         MOVEQ   R2,#0                   ; if we were called with R2='TASK' then
         STREQ   R2,[sp,#(2+1+8)*4]      ; ensure R2=0 on return - taskhandle stacked below r2
 08