Fix detecting whether sources are configured (borgbase#1613)

Fixes borgbase#1463. This makes the code count the database rows for the current profile only. 
Previously one could run a backup without any sources when one had sources configured in a different profile.

* src/vorta/borg/create.py (BorgCreateJob.prepare)

src/vorta/borg/create.py

@@ -77,7 +77,7 @@ def prepare(cls, profile):
         else:
             ret['ok'] = False  # Set back to False, so we can do our own checks here.
 
-        n_backup_folders = SourceFileModel.select().count()
+        n_backup_folders = SourceFileModel.select().where(SourceFileModel.profile == profile).count()
 
         # cmd options like `--paths-from-command` require a command
         # that is appended to the arguments