stake-pool: Add checked math to big vec calc (#2524)

Author: joncinque

stake-pool/program/src/big_vec.rs

@@ -90,7 +90,10 @@ impl<'data> BigVec<'data> {
         len: usize,
     ) -> Result<Vec<&'data mut T>, ProgramError> {
         let vec_len = self.len();
-        if skip + len > vec_len as usize {
+        let last_item_index = skip
+            .checked_add(len)
+            .ok_or(ProgramError::AccountDataTooSmall)?;
+        if last_item_index > vec_len as usize {
             return Err(ProgramError::AccountDataTooSmall);
         }