trace: fix coverity warnings

[alfredh]

Fixes around 4 warnings from Coverity 2023.6

145int re_trace_init(const char *json_file)
146{
147        int err = 0;
148
149#ifndef RE_TRACE_ENABLED
150        return 0;
151#endif
152
     	CID 469999: Structurally dead code (UNREACHABLE) [[select issue](https://scan8.scan.coverity.com/defectInstanceId=13589814&fileInstanceId=88291432&mergedDefectId=469999)]
153        if (!json_file)
154                return EINVAL;
155
156        if (re_atomic_rlx(&trace.init))
157                return EALREADY;