common: check file existence before opening manifest
common: check file existence before opening manifest

This fixes a problem when there is a broken symlink with the name
AndroidManifest.xml which will then lead to a crash when trying to open
it for trying to remove the debuggable flag.

f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver