MBR: Fix issue with MBR init asserting on bad partition size

[geky]

Description

With the assert, applications were unable to check the corrupted MBR and correct it, so invalid data on a block device would prevent an application from recovering.

cc @marcuschangarm, @dlfryar, @ARMmbed/mbed-os-storage

Pull request type

[x] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[cmonr]

👍

[cmonr]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2427
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7299/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2056
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7299/

[mbed-ci]

Test : SUCCESS

Build number : 2207
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7299/2207

[geky]

Hey @dannybenor, @davidsaada, would one of you be able to review this pr. It's small but a part of the pr process.

[geky]

@davidsaada Thanks!

[marcuschangarm]

Thank you all! 🎉