TDBStore: Unlock the master mutex even after garbage_collect()

[SeppoTakalo]

Description

Summary of change

TDBStore::set() was leaving a mutex locked in case it failed to write a data, and then started a garbage collection phase.

Fixed the sequence so that mutex is always unlocked, regardless of whether we run the GC or not.

Fixes #11723

---

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)

---

Test results

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

---

Reviewers

@VeijoPesonen

[VeijoPesonen]

Looks good to me

[mbed-ci]

Test run: SUCCESS

Summary: 11 of 11 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

Restarted travis, has not reported back