dtc: Check return value from fwrite()

[Jon Loeliger]

> There's one place in flattree.c where we currently ignore the return
> value from fwrite().  On some gcc/glibc versions, where fwrite() is
> declared with attribute warn_unused_result, this causes a warning.
> 
> This patch fixes the warning, by checking the fwrite() result.
> 
> Signed-off-by: David Gibson <david at gibson.dropbear.id.au>

Applied.

Thanks,
jdl