[PATCH] fix crash for sed "script" /\(\)/x

[Paolo Bonzini]

An empty subexpression in the presence of REG_NOSUB may cause a CONCAT 
node to have a NULL children, and this would crash in calc_first.

The attached patch cures it by not optimizing the empty subexpression in 
the first place.  It ought not to be very common, so we do not lose much.

Paolo
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