bnxt_en: Fix NULL pointer dereference

In the Linux kernel, the following vulnerability has been resolved: bnxt_en: Fix NULL pointer dereference PCIe errors detected by a Root Port or Downstream Port cause error recovery services to run on all subordinate devices regardless of administrative state. The .error_detected() callback, bnxt_io_error_detected(), disables and synchronizes IRQs via bnxt_disable_int_sync(), which calls bnxt_cp_num_to_irq_num() to map completion rings to IRQs using bp->bnapi. Since bp->bnapi is allocated on NIC open and freed on NIC close, PCIe error recovery on a closed NIC can dereference a NULL pointer. Check if bp->bnapi is NULL before disabling and synchronizing IRQs.