Add invalid data case to tdbstore ram init

[amq]

Summary of changes

This prevents the following crash on tdbstore init:

++ MbedOS Error Info ++
Error Status: 0x80FF011B Code: 283 Module: 255
Error Message: TDBSTORE: Unable to build RAM table at init
Location: 0x364E7
Error Value: 0x0
Current Thread: main Id: 0x20002818 Entry: 0x3162B StackSize: 0x1000 StackMem: 0x20003E28 SP: 0x20004CEC 
For more info, visit: https://mbed.com/s/error?error=0x80FF011B&tgt=EFM32PG12_STK3402
-- MbedOS Error Info --

The most probable cause is powering off the module right when the next value of a key is written and provided that this key and its value is the last thing that fits into the current memory block.

Impact of changes

None.

Migration actions required

None.

Documentation

None.

---

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

@SeppoTakalo

---

[ciarmcom]

@amq, thank you for your changes.
@SeppoTakalo @ARMmbed/mbed-os-storage @ARMmbed/mbed-os-maintainers please review.

[amq]

@SeppoTakalo, thanks, I've applied the change based on your review.

[0xc0170]

CI started

[mbed-ci]

Test run: SUCCESS

Summary: 6 of 6 test jobs passed
Build number : 1
Build artifacts