FileBase: Fix unsetting of default when destroyed

[kjbracey]

Description

Code that should unset a FileBase from being the default when it is destroyed was broken by a == instead of = typo.

Pull request type

[X] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Breaking change

[adbridge]

Looks good to me

[NirSonnenschein]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 3076
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8052/

Triggering tests

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

[mbed-ci]

Test : SUCCESS

Build number : 2867
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8052/2867

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2689
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8052/