sign_verify_detached: guard against size

The code didn't properly do a size check on a detached signature. Now it
does.

While here, fix a problem with EQC tests, for the same piece of code.

c_src/sign.c

@@ -468,6 +468,10 @@ enacl_crypto_sign_verify_detached(ErlNifEnv *env, int argc,
     return enif_make_badarg(env);
   }
 
+  if (sig.size != crypto_sign_BYTES) {
+    return enif_make_badarg(env);
+  }
+
   if (pk.size != crypto_sign_PUBLICKEYBYTES) {
     return enif_make_badarg(env);
   }