dtc: clean up grow_data_for()

Change the grow_data_for function to copy struct data and
modifiy the fields it is updating instead of storing all
fields individually to a stack allocated struct.

This reduces maintence for future enhancements as now all
instances of struct data are created by modifying a copy
of an existing struct data or directly copying empty_data.

Signed-off-by: Milton Miller <miltonm@bga.com>

data.c

@@ -57,6 +57,8 @@ struct data data_grow_for(struct data d, int xlen)
 	if (xlen == 0)
 		return d;
 
+	nd = d;
+
 	newsize = xlen;
 
 	while ((d.len + xlen) > newsize)
@@ -64,8 +66,6 @@ struct data data_grow_for(struct data d, int xlen)
 
 	nd.asize = newsize;
 	nd.val = xrealloc(d.val, newsize);
-	nd.len = d.len;
-	nd.refs = d.refs;
 
 	assert(nd.asize >= (d.len + xlen));