musig: explicitly panic when given an empty slice of pubkeys to aggregate

The existing code panics somewhere in ffi.rs trying to do pointer
alignment stuff, and is only not UB by accident. Guard this with an
explicit panic.

Author: apoelstra

src/musig.rs

@@ -314,6 +314,10 @@ impl KeyAggCache {
     ///
     /// Panics if an empty slice of pubkeys is provided.
     pub fn new<C: Verification>(secp: &Secp256k1<C>, pubkeys: &[&PublicKey]) -> Self {
+        if pubkeys.is_empty() {
+            panic!("Cannot aggregate an empty slice of pubkeys");
+        }
+
         let cx = secp.ctx().as_ptr();
 
         let mut key_agg_cache = MaybeUninit::<ffi::MusigKeyAggCache>::uninit();