virt-aa-helper: Fix return value of add_file_path

Commit a8853344 added this
function and wrapped vah_add_file in it. vah_add_file may
return -1, 0, 1. It returns 1 in case the call to valid_path
detects a restricted file. The original code treated a return
value != 0 as error. The refactored code treats a return
value < 0 as error. This triggers segfault in virt-aa-helper
and breaks virt-aa-helper-test for the restricted file tests.

Make sure that add_file_path returns -1 on error.

src/security/virt-aa-helper.c

@@ -818,6 +818,9 @@ add_file_path(virDomainDiskDefPtr disk,
         ret = vah_add_file(buf, path, "r");
     }
 
+    if (ret != 0)
+        ret = -1;
+
     return ret;
 }