Unchecked return value in fdt_add_subnode_namelen() about dtc

[dgibson]

We can't use assert() in libfdt, because we it needs to be usable in very constrained environments (like bootloaders) which might not have an assert() (or if they do it might not work like the libc one). Our rough equivalent to assert()s is to return FDT_ERR_INTERNAL.

I'm not sure why our own Coverity didn't pick this one up, but in any case I've made what should be a fix in a somewhat different way.