Reject empty generation strings.

Reported by Hanno Böck <hanno@hboeck.de>

Reviewed-by: Rich Salz <rsalz@openssl.org>

crypto/asn1/asn1_gen.c

diff --git a/crypto/asn1/asn1_gen.c b/crypto/asn1/asn1_gen.c
index 36fc218..cee3749 100644
--- a/crypto/asn1/asn1_gen.c
+++ b/crypto/asn1/asn1_gen.c
@@ -167,8 +167,10 @@
     asn1_tags.imp_class = -1;
     asn1_tags.format = ASN1_GEN_FORMAT_ASCII;
     asn1_tags.exp_count = 0;
-    if (CONF_parse_list(str, ',', 1, asn1_cb, &asn1_tags) != 0)
+    if (CONF_parse_list(str, ',', 1, asn1_cb, &asn1_tags) != 0) {
+        *perr = ASN1_R_UNKNOWN_TAG;
         return NULL;
+    }
 
     if ((asn1_tags.utype == V_ASN1_SEQUENCE)
         || (asn1_tags.utype == V_ASN1_SET)) {
@@ -298,7 +300,7 @@
     int tmp_tag, tmp_class;
 
     if (elem == NULL)
-        return 0;
+        return -1;
 
     for (i = 0, p = elem; i < len; p++, i++) {
         /* Look for the ':' in name value pairs */