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.

Author: wtarreau

src/trace.c

@@ -1008,33 +1008,6 @@ int trace_parse_cmd(const char *arg_src, char **errmsg)
 	char *arg, *oarg;
 	char *saveptr;
 
-	if (arg_src) {
-		if (strcmp(arg_src, "help") == 0) {
-			memprintf(errmsg,
-			  "-dt activates traces on stderr output via the command-line.\n"
-			  "Without argument, all registered trace sources are activated with error level as filter.\n"
-			  "A list can be specified as argument to configure several trace sources with comma as separator.\n"
-			  "Each entry can contains the trace name, a log level and a verbosity using colon as separator.\n"
-			  "Every fields are optional and can be left empty, or with a colon to specify the next one.\n\n"
-			  "An empty name or the alias 'all' will activate all registered sources.\n"
-			  "Verbosity cannot be configured in this case except 'quiet' as their values are specific to each source.\n\n"
-			  "Examples:\n"
-			  "-dt           activate every sources on error level\n"
-			  "-dt all:user  activate every sources on user level\n"
-			  "-dt h1        activate HTTP/1 traces on error level\n"
-			  "-dt h2:data   activate HTTP/2 traces on data level\n"
-			  "-dt quic::clean,qmux::minimal\n    activate both QUIC transport and MUX traces on error level with their custom verbosity\n");
-			return -1;
-		}
-
-		/* keep a copy of the ptr for strtok */
-		oarg = arg = strdup(arg_src);
-		if (!arg) {
-			memprintf(errmsg, "Can't allocate !");
-			return -2;
-		}
-	}
-
 	if (!arg_src) {
 		/* No trace specification, activate all sources on error level. */
 		struct trace_source *src = NULL;
@@ -1044,6 +1017,31 @@ int trace_parse_cmd(const char *arg_src, char **errmsg)
 		return 0;
 	}
 
+	if (strcmp(arg_src, "help") == 0) {
+		memprintf(errmsg,
+		          "-dt activates traces on stderr output via the command-line.\n"
+		          "Without argument, all registered trace sources are activated with error level as filter.\n"
+		          "A list can be specified as argument to configure several trace sources with comma as separator.\n"
+		          "Each entry can contains the trace name, a log level and a verbosity using colon as separator.\n"
+		          "Every fields are optional and can be left empty, or with a colon to specify the next one.\n\n"
+		          "An empty name or the alias 'all' will activate all registered sources.\n"
+		          "Verbosity cannot be configured in this case except 'quiet' as their values are specific to each source.\n\n"
+		          "Examples:\n"
+		          "-dt           activate every sources on error level\n"
+		          "-dt all:user  activate every sources on user level\n"
+		          "-dt h1        activate HTTP/1 traces on error level\n"
+		          "-dt h2:data   activate HTTP/2 traces on data level\n"
+		          "-dt quic::clean,qmux::minimal\n    activate both QUIC transport and MUX traces on error level with their custom verbosity\n");
+		return -1;
+	}
+
+	/* keep a copy of the ptr for strtok */
+	oarg = arg = strdup(arg_src);
+	if (!arg) {
+		memprintf(errmsg, "Can't allocate trace source!");
+		return -2;
+	}
+
 	while ((str = strtok_r(arg, ",", &saveptr))) {
 		struct trace_source *src = NULL;
 		char *field, *name;