xentrace: undeadify invalid option argument handling

Apparently it's always been like this.

Coverity-ID: 1056153
Signed-off-by: Matthew Daley <mattjd@gmail.com>
Reviewed-by: George Dunlap <george.dunlap@eu.citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

tools/xentrace/xentrace.c

@@ -858,10 +858,11 @@ long sargtol(const char *restrict arg, int base)
 
 
     return val;
+
 invalid:
-    return 0;
     fprintf(stderr, "Invalid option argument: %s\n\n", arg);
     usage();
+    return 0; /* not actually reached */
 }
 
 /* convert the argument string pointed to by arg to a long int representation */