libfdt: Remove superfluous NUL character from overlay symbols

The symbol path already ends with a NUL character (something which is
actually checked by the code) and this NUL is included in
rel_path_len, so there is no need to add a second one.

This change fixes incorrect display in "fdt list /__symbols" after
applying an overlay with symbols.

Signed-off-by: Marcus Comstedt <marcus@mc.pp.se>
Reviewed-by: Simon Glass <sjg@chromium.org>