Check b.len() < Self::FIXED_SIZE, not b.len() < 4

The original thinking was that if len(b) < FIXED_SIZE, then that would
be picked up later by the "invalid member length" check. In that case,
this only really *needs* to make sure that the check after this for
"wrong fourcc" wouldn't fail. But if len(b) < FIXED_SIZE, it really is
an invalid size, and should get that error message before even starting
to validate its other qualities.

veilid-core/src/veilid_api/types/dht/schema/smpl.rs

@@ -96,7 +96,7 @@ impl DHTSchemaSMPL {
 impl TryFrom<&[u8]> for DHTSchemaSMPL {
     type Error = VeilidAPIError;
     fn try_from(b: &[u8]) -> Result<Self, Self::Error> {
-        if b.len() < 4 {
+        if b.len() < Self::FIXED_SIZE {
             apibail_generic!("invalid size");
         }
         if &b[0..4] != &Self::FCC {