script: make it work even if fd 0 is closed

Testcase:
script -q -c "echo hey" /dev/null 0>&-

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/getpty.c

@@ -16,7 +16,7 @@ int FAST_FUNC xgetpty(char *line)
 
 #if ENABLE_FEATURE_DEVPTS
 	p = open("/dev/ptmx", O_RDWR);
-	if (p > 0) {
+	if (p >= 0) {
 		grantpt(p); /* chmod+chown corresponding slave pty */
 		unlockpt(p); /* (what does this do?) */
 # ifndef HAVE_PTSNAME_R