littlefs2: Properly init class variables

[multiplemonomials]

Summary of changes

Noticed this bug on the Arduino bug tracker which is happening due to this->_bd being uninitialized in LittleFileSystem2, leading to undefined behavior if unmount() is called after constructing the class.

Impact of changes

Should no longer crash in this situation.

Migration actions required

Documentation

None

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Pull request type

[X] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[X] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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