Fix bug: xref should be stopped after being run

Xref not being stopped caused a crash in subsequent xref runs.

src/rebar_xref.erl

@@ -79,6 +79,9 @@ xref(Config, _) ->
     %% Restore the original code path
     true = code:set_path(OrigPath),
 
+    %% Stop xref
+    stopped = xref:stop(xref),
+
     ok.
 
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