modbus: free eve thread context on deinit

Was triggering ASAN leak detection.

src/output-json-modbus.c

@@ -132,6 +132,7 @@ static TmEcode JsonModbusLogThreadDeinit(ThreadVars *t, void *data)
     if (thread == NULL) {
         return TM_ECODE_OK;
     }
+    FreeEveThreadCtx(thread->ctx);
     SCFree(thread);
     return TM_ECODE_OK;
 }