lib/nvme: fix heap-use-after-free on pqpair's shadow_doorbell

pqpair shadow_doorbell point to a memory space allocated by ctrlr, when
ctrlr is disconnected before qpair is deleted, `pqpair->shadow_doorbell`
try to change value of already released memory.

```
==64530==ERROR: AddressSanitizer: heap-use-after-free on address 0x20003041a008 at pc 0x7f66758f8e4d bp 0x7f666f6f84e0 sp 0x7f666f6f84d8
WRITE of size 4 at 0x20003041a008 thread T3 (reactor_2)
```
Signed-off-by: Sebastian Brzezinka <sebastian.brzezinka@intel.com>
Change-Id: I9c8b079dab1a34d34f41fc43a0db6ec35f40cc17
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/19703


Reviewed-by: Jim Harris <jim.harris@gmail.com>
Reviewed-by: Aleksey Marchuk <alexeymar@nvidia.com>
Reviewed-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Ben Walker <ben@nvidia.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Michael Haeuptle <michaelhaeuptle@gmail.com>