flamenco, vm: fix zero-sized memcmp (#7169)

Author: topointon-jump

src/flamenco/vm/syscall/fd_vm_syscall_cpi_common.c

@@ -212,7 +212,8 @@ VM_SYCALL_CPI_UPDATE_CALLEE_ACC_FUNC( fd_vm_t *                          vm,
         return -1;
       }
     } else if( FD_UNLIKELY( caller_account->serialized_data_len!=fd_borrowed_account_get_data_len( &callee_acc ) ||
-      memcmp( fd_borrowed_account_get_data( &callee_acc ), caller_account->serialized_data, caller_account->serialized_data_len ) ) ) {
+      ( caller_account->serialized_data_len &&
+        memcmp( fd_borrowed_account_get_data( &callee_acc ), caller_account->serialized_data, caller_account->serialized_data_len ) ) ) ) {
       /* https://github.com/anza-xyz/agave/blob/v3.0.4/syscalls/src/cpi.rs#L1117-L1119 */
       FD_VM_ERR_FOR_LOG_INSTR( vm, err );
       return -1;