evmgr: Fix a "maybe uninitialized" issue

The warning maybe-uninitialized is emited only
when the optimisation level is greater than 0.
For this reason, the error was not discovered
development process. The build config is tuned
to detect that error and the error is fixed.

Change-Id: I14c8ffe6daa3d498268cfadeab20300895b3360e
Signed-off-by: José Bollo <jose.bollo@iot.bzh>