Fix image read and segfault in fwu-tool

Fix fread return value check, to only report errors on negative values.
Add NULL check to fwu_app destructor to avoid segfault on exit.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: Ia5924dc81866013e1edeef2aa4105e6283b157f5
2 files changed