PR17737 - already initialized data variables.

[Paul Thomas]

This causes an ICE:

	integer	i
	data i/0/
	data i/0/
	end

This is not standard F95.  ifc warns of this but takes the last value.

My inclination is to follow suite; in fact, in some ways it is easier to 
do that to catch it in match_data.  How about warning unless -std=f95, 
in which case error?

Paul T