evaluate: Reject quoted strings containing only wildcard

Fix for an assertion fail when trying to match against an all-wildcard
interface name:

| % nft add rule t c iifname '"*"'
| nft: expression.c:402: constant_expr_alloc: Assertion `(((len) + (8) - 1) / (8)) > 0' failed.
| zsh: abort      nft add rule t c iifname '"*"'

Fix this by detecting the string in expr_evaluate_string() and returning
an error message:

| % nft add rule t c iifname '"*"'
| Error: All-wildcard strings are not supported
| add rule t c iifname "*"
|                      ^^^

While being at it, drop the 'datalen >= 1' clause from the following
conditional as together with the added check for 'datalen == 0', all
possible other values have been caught already.

0 files changed, 0 insertions, 0 deletions