tools: fdtgrep: Fix handle leak

The handle "fd" was created in fdtgrep.c:708 by calling the
 "open" function and is lost in fdtgrep.c:716 and fdtgrep.c:723.
 Close file descriptor 'fd' before exiting with an error from function
 utilfdt_read_err_len(const char *filename, char **buffp, off_t *len).

Fixes: 1043d0a0 ("fdt: Add fdtgrep tool")
Signed-off-by: Mikhail Ilin <ilin.mikhail.ol@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>