MINOR: cfgparse-ssl: avoid a possible crash on OOM in ssl_bind_parse_npn()

Upon out of memory condition at boot, we could possibly crash when
parsing the "npn" bind line keyword since it's used unchecked. There's
no real need to backport this though it will not hurt.
diff --git a/src/cfgparse-ssl.c b/src/cfgparse-ssl.c
index 4f999e1..75af0e8 100644
--- a/src/cfgparse-ssl.c
+++ b/src/cfgparse-ssl.c
@@ -1016,6 +1016,11 @@
 	 */
 	conf->npn_len = strlen(args[cur_arg + 1]) + 1;
 	conf->npn_str = calloc(1, conf->npn_len + 1);
+	if (!conf->npn_str) {
+		memprintf(err, "out of memory");
+		return ERR_ALERT | ERR_FATAL;
+	}
+
 	memcpy(conf->npn_str + 1, args[cur_arg + 1], conf->npn_len);
 
 	/* replace commas with the name length */