Fix for crashes loading sprites on RISC OS 3.10

A register fumble (revision 4.43 of this file) meant the pointer to the error block from OS_ReadSysInfo not existing was being cached as IRQsema, so later any attempts to resize the RMA would be skipped believing it to be unsafe (because the error pointer would point to an error block whose first word is mostly non zero).
Tested on a real A5000 and emulated via ArcEm.

s/Wimp01

@@ -3404,7 +3404,7 @@ gotwork
         MOVVS   R2,#0
         CMP     R2,#0
         LDREQ   R2,=Legacy_IRQsema
-        STR     R0,ptr_IRQsema
+        STR     R2,ptr_IRQsema
         MOV     R0,#6
         MOV     R2,#OSRSI6_DomainId
         SWI     XOS_ReadSysInfo