of/fdt: Don't copy garbage after "/" in root node path

The root node path must be internally converted to "/", or various
pieces of code looking for it that way will fail. The code to do
that however had a bug where we might incorrectly append pieces
of the original path from the fdt to the "/".

We should probably add a proper dedicated accessor for the root node
but in the meantime this patch should fix it.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>

drivers/of/fdt.c

@@ -186,6 +186,7 @@ static unsigned long unflatten_dt_node(struct boot_param_header *blob,
 			 */
 			fpsize = 1;
 			allocl = 2;
+			l = 0;
 		} else {
 			/* account for '/' and path size minus terminal 0
 			 * already in 'l'