MacOS X port: do not free argv if it was not allocated.

Darwin revealed a bug in our free_proc_title() function, which was
added to procitle.c to work well with valgrind: we free argv pointer
even if it wasn't allocated (none of PS_USE_CLOBBER_ARGV/PS_USE_CHANGE_ARGV
was set).

Don't free memory if it wasn't allocated.

Another issue is understanding why Darwin doesn't define
PS_USE_SETPROCTITLE and why our FreeBSD port didn't hit this bug.

third_party/proctitle.c

@@ -256,10 +256,12 @@ free_proc_title(int argc, char **argv)
 		free(environ[i]);
 	free(environ);
 	environ = save_environ;
-#endif
+#endif /* PS_USE_CLOBBER_ARGV */
+#if defined(PS_USE_CHANGE_ARGV) || defined(PS_USE_CLOBBER_ARGV)
 	for (i = 0; i < argc; i++)
 		free(argv[i]);
 	free(argv);
+#endif /* PS_USE_CHANGE_ARGV or PS_USE_CLOBBER_ARGV */
 }
 
 void