Fix for null pointer dereference & data abort on swapping to no toolsprites

This specific case failed when the topmost window in the window stack (ie. the first to be attempted to be redrawn on reentering the desktop) had the focus and thus attempts to enable Ursula tinting.
Though a check was made to see if the first sprite (from which the tint is derived) was present, no check was made to see if the tool_list was present => dereferenced NULL pointer.
Retagged as Wimp-5_29.

s/Wimp02

@@ -8841,8 +8841,9 @@ no_thin_outline
 ;
 forcetint_on EntryS "R0,R2"
         LDR     R2, tool_list
-        LDR     R2, [R2]
-        TEQ     R2, #0                  ; don't bother if there aren't any tools
+        TEQ     R2, #0
+        LDRNE   R2, [R2, #tool_back]
+        TEQNE   R2, #0                  ; don't bother if there aren't any tools
         EXITS   EQ
 
         LDRB    R0, tinted_window
@@ -8858,8 +8859,9 @@ forcetint_on EntryS "R0,R2"
 ;
 forcetint_off EntryS "R2"
         LDR     R2, tool_list
-        LDR     R2, [R2]
-        TEQ     R2, #0                  ; don't bother if there aren't any tools
+        TEQ     R2, #0
+        LDRNE   R2, [R2, #tool_back]
+        TEQNE   R2, #0                  ; don't bother if there aren't any tools
         EXITS   EQ
 
         LDRB    R14, tinted_tool