CVE-2023-53366 block: be a bit more careful in checking for NULL bdev while polling

[
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "unaffected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "block/blk-core.c"
    ],
    "versions": [
      {
        "version": "be4d234d7aebbfe0c233bc20b9cdef7ab3408ff4",
        "lessThan": "1af0bdca03f367874da45d6cbe05fa05b90b1439",
        "status": "affected",
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      },
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      },
      {
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    ]
  },
  {
    "product": "Linux",
    "vendor": "Linux",
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    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "block/blk-core.c"
    ],
    "versions": [
      {
        "version": "5.15",
        "status": "affected"
      },
      {
        "version": "0",
        "lessThan": "5.15",
        "status": "unaffected",
        "versionType": "semver"
      },
      {
        "version": "6.1.16",
        "lessThanOrEqual": "6.1.*",
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      },
      {
        "version": "6.2.3",
        "lessThanOrEqual": "6.2.*",
        "status": "unaffected",
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      },
      {
        "version": "6.3",
        "lessThanOrEqual": "*",
        "status": "unaffected",
        "versionType": "original_commit_for_fix"
      }
    ]
  }
]