bug RSSA backend

[Matthew Fluet]

The following program fails to terminate on second execution with both
-native true and -native false.  I think something is messed up in
MLton.Random.alphaNumString, because it seems to always return EEEEEE.

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val (w, out) = MLton.TextIO.mkstemp "temp"
val _ = print (w^"\n")
val _ = TextIO.output (out, "ABCDEF\n")
val _ = TextIO.closeOut out
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