RFR(XS): 8161907: adlc: Fix crash in cisc_spill_match if _rChild == NULL"

[Lindenmaier, Goetz]

Hi,

Please review and sponsor this tiny, rather obvious change:
http://cr.openjdk.java.net/~goetz/wr16/8161907-adlc/

_rChild is never checked for NULL, but it is dereferenced.
We have a rule that causes a crash here.

Best regards,
  Goetz.

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