of/fdt: NULL-terminate the root node path

Commit 509b7455 (of/fdt: Don't copy garbage after "/" in root node path)
sets the path length to 0 to ignore any garbage after "/" in the root
node path. This has the side effect of also ignoring '\0' at the end of
the root node path. This patch sets the ignores the garbage by setting
the last character to '\0' and length to 1.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>