Don't segfault if setproctitle(3) is called with NULL initially.

The old buffer was not being initialized and a later str*() op on
it would cause a crash if it wasn't initialized by a previous
call to setproctitle(3) with an actual string.

Noticed by: Ashley Penney <ashp@unloved.org>

lib/libc/gen/setproctitle.c

@@ -86,6 +86,7 @@ setproctitle(const char *fmt, ...)
 		obuf = malloc(SPT_BUFSIZE);
 		if (obuf == NULL)
 			return;
+		*obuf = '\0';
 	}
 
 	va_start(ap, fmt);