parser: reject zero-length interface names

parser: reject zero-length interface names

device "" results in an assertion during evaluation.
Before:
nft: expression.c:426: constant_expr_alloc: Assertion `(((len) + (8) - 1) / (8)) > 0' failed.
After:
zero_length_devicename_assert:3:42-49: Error: you cannot set an empty interface name
type filter hook ingress device""lo" priority -1
                         ^^^^^^^^
Signed-off-by: Florian Westphal <fw@strlen.de>