BUILD: trace: fix warning on null dereference

BUILD: trace: fix warning on null dereference

Since a recent change on trace, the following compilation warning may
occur :
  src/trace.c: In function ‘trace_parse_cmd’:
  src/trace.c:865:33: error: potential null pointer dereference [-Werror=null-dereference]
    865 |                         for (nd = src->decoding; nd->name && nd->desc; nd++)
        |                              ~~~^~~~~~~~~~~~~~~

Fix this by rearranging code path to better highlight that only "quiet"
verbosity is allowed if no trace source is specified.

This was detected with GCC 14.1.