Pass argument into hashtable_new
@8915d8d87 and @4471b7350 modified "usetxn" parameter in
"hashtable_new" scope (was a global variable before).
But the callers of this function don't pass argument into.
Thus, "usetxn" acts as an uninitialized auto variable.
Fixes: https://pagure.io/389-ds-base/issue/50057