Fix seg fault in ASN1_generate_v3/ASN1_generate_nconf

Reviewed-by: Dr. Stephen Henson <steve@openssl.org>

crypto/asn1/asn1_gen.c

diff --git a/crypto/asn1/asn1_gen.c b/crypto/asn1/asn1_gen.c
index 16a1b5a..0e1cc08 100644
--- a/crypto/asn1/asn1_gen.c
+++ b/crypto/asn1/asn1_gen.c
@@ -353,6 +353,10 @@
         break;
 
     case ASN1_GEN_FLAG_FORMAT:
+        if(!vstart) {
+            ASN1err(ASN1_F_ASN1_CB, ASN1_R_UNKNOWN_FORMAT);
+            return -1;
+        }
         if (!strncmp(vstart, "ASCII", 5))
             arg->format = ASN1_GEN_FORMAT_ASCII;
         else if (!strncmp(vstart, "UTF8", 4))