Compile and warning fixes

programs/mango-v4/src/error.rs

@@ -12,4 +12,6 @@ pub enum MangoError {
     UnknownOracleType,
     #[msg("")]
     InvalidMarginTradeTargetCpiProgram,
+    #[msg("")]
+    HealthMustBePositive,
 }

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

@@ -33,7 +33,7 @@ pub fn margin_trade(ctx: Context<MarginTrade>, cpi_data: Vec<u8>) -> Result<()>
     // abuse this ix to do unwanted changes
     for cpi_ai in cpi_ais {
         require!(
-            *ctx.remaining_accounts[active_len].key != Mango::id(),
+            cpi_ai.key() != Mango::id(),
             MangoError::InvalidMarginTradeTargetCpiProgram
         );
     }