Fuzzing 1/2: Negative values accepted for string-functions.

[Steve Kemp]

The following program causes a panic:

   0REM
   24734394666510340REM s t I s
   30REM
   40PRINT" ",CODE"*","n"
   50PRINT" ",CODE" ","n"
   100LET A="Steve��
             "
   110LET L=(LEN A)- 1
   120FOR I=0TO L
   130LET X=MID$ A,I,-999172243PRINT" ",I,"s",X,"e",CODE X,"n"
   150000000f