BUILD: trace: silence a bogus build warning at -Og

BUILD: trace: silence a bogus build warning at -Og

gcc-13.3 at -Og emits an incorrect build warning in trace.c about a
possibly initialized variable:

  In file included from include/haproxy/api.h:35,
                   from src/trace.c:22:
  src/trace.c: In function 'trace_parse_cmd':
  include/haproxy/bug.h:431:17: warning: 'arg' may be used uninitialized [-Wmaybe-uninitialized]
    431 |                 free(*__x);                                             \
        |                 ^~~~~~~~~~
  src/trace.c:1136:9: note: in expansion of macro 'ha_free'
   1136 |         ha_free(&oarg);
        |         ^~~~~~~
  src/trace.c:1008:15: note: 'arg' was declared here
   1008 |         char *arg, *oarg;
        |               ^~~

The warning is obviously wrong since the field is initialized in one of
the two branches of an "if" whose complementary one returns. But the
compiler doesn't seem to see this because the if is in fact two ifs each
with an opposite condition: "if (arg_src)" then "if (!arg_src)". Let's
just move upwards the default one that returns and eliminate the other
one. Reading the diff with "git diff -b" better shows the tiny change.

It could be backported to 3.0.