evaluate: verify named map is actually a map

evaluate: verify named map is actually a map

# nft add set filter test { type ipv4_addr; }
# nft filter input ip daddr vmap @test

Before:

<cmdline>:0:0-32: Error: Could not process rule: Invalid argument
filter input ip daddr vmap @test
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

After:

<cmdline>:1:28-32: Error: Expression is not a map
filter input ip daddr vmap @test
                           ^^^^^

Signed-off-by: Patrick McHardy <kaber@trash.net>