Reset ICU converter pointer after closing it.

* The pointer to the ICU converter that's kept in TLS needs to be
  reset (to NULL) immediately after closing it, as opening a new
  container may fail, leaving a dangling pointer to a now closed
  container in the TLS value.

Fixes #8361 & the userland cause of #8430.

src/system/libroot/add-ons/icu/ICUCategoryData.cpp

@@ -125,6 +125,7 @@ ICUCategoryData::_GetConverter(UConverter*& converterOut)
 		// charset no longer matches the converter, we need to dump it and
 		// create a new one
 		ucnv_close(tlsValue->converter);
+		tlsValue->converter = NULL;
 	}
 
 	// create a new converter for the current charset