Add explicit self-liquidation checks (#427)

Just for safety, the other conditions and borrowing pattern would
already make it impossible.

programs/mango-v4/src/instructions/perp_liq_base_position.rs

@@ -45,6 +45,7 @@ pub fn perp_liq_base_position(
 ) -> Result<()> {
     let group_pk = &ctx.accounts.group.key();
 
+    require_keys_neq!(ctx.accounts.liqor.key(), ctx.accounts.liqee.key());
     let mut liqor = ctx.accounts.liqor.load_full_mut()?;
     // account constraint #1
     require!(

programs/mango-v4/src/instructions/perp_liq_quote_and_bankruptcy.rs

@@ -84,12 +84,6 @@ pub fn perp_liq_quote_and_bankruptcy(
 ) -> Result<()> {
     let mango_group = ctx.accounts.group.key();
 
-    // Cannot settle with yourself
-    require!(
-        ctx.accounts.liqor.key() != ctx.accounts.liqee.key(),
-        MangoError::SomeError
-    );
-
     let (perp_market_index, settle_token_index) = {
         let perp_market = ctx.accounts.perp_market.load()?;
         (
@@ -98,6 +92,7 @@ pub fn perp_liq_quote_and_bankruptcy(
         )
     };
 
+    require_keys_neq!(ctx.accounts.liqor.key(), ctx.accounts.liqee.key());
     let mut liqee = ctx.accounts.liqee.load_full_mut()?;
     let mut liqor = ctx.accounts.liqor.load_full_mut()?;
     // account constraint #1

programs/mango-v4/src/instructions/perp_settle_pnl.rs

@@ -57,8 +57,9 @@ pub struct PerpSettlePnl<'info> {
 
 pub fn perp_settle_pnl(ctx: Context<PerpSettlePnl>) -> Result<()> {
     // Cannot settle with yourself
-    require!(
-        ctx.accounts.account_a.key() != ctx.accounts.account_b.key(),
+    require_keys_neq!(
+        ctx.accounts.account_a.key(),
+        ctx.accounts.account_b.key(),
         MangoError::CannotSettleWithSelf
     );
 

programs/mango-v4/src/instructions/token_liq_bankruptcy.rs

@@ -84,6 +84,8 @@ pub fn token_liq_bankruptcy(
     let (bank_ais, health_ais) = &ctx.remaining_accounts.split_at(liab_mint_info.num_banks());
     liab_mint_info.verify_banks_ais(bank_ais)?;
 
+    require_keys_neq!(ctx.accounts.liqor.key(), ctx.accounts.liqee.key());
+
     let mut liqor = ctx.accounts.liqor.load_full_mut()?;
     // account constraint #1
     require!(

programs/mango-v4/src/instructions/token_liq_with_token.rs

@@ -47,6 +47,7 @@ pub fn token_liq_with_token(
     let mut account_retriever = ScanningAccountRetriever::new(ctx.remaining_accounts, group_pk)
         .context("create account retriever")?;
 
+    require_keys_neq!(ctx.accounts.liqor.key(), ctx.accounts.liqee.key());
     let mut liqor = ctx.accounts.liqor.load_full_mut()?;
     // account constraint #1
     require!(