Fix seg fault in ASN1_generate_v3/ASN1_generate_nconf

(Imported from upstream's ac5a110621)

Change-Id: I1a0518447716a93038bd95d40abef1d9fb4e5f72
Reviewed-on: https://boringssl-review.googlesource.com/4004
Reviewed-by: Adam Langley <agl@google.com>

crypto/x509/asn1_gen.c

@@ -373,6 +373,11 @@ static int asn1_cb(const char *elem, int len, void *bitstr)
 		break;
 
 		case ASN1_GEN_FLAG_FORMAT:
+		if (!vstart)
+			{
+			OPENSSL_PUT_ERROR(ASN1, 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))