Fix string sort

SMILEBASIC/TEST.txt

@@ -39,6 +39,8 @@ DEF SORTTEST
  ASSERT__ A[3]==0,"RSORT"
  ASSERT__ B$[3]=="3","RSORT"
  ASSERT__ C$[3]=="D","RSORT"
+ RSORT B$
+ ASSERT__ B$[3]=="0","RSORT2"
  SORTTEST2
 END
 DEF SORTTEST2