let's use an empty list element to signal an empty sequence

src/pk/asn1/der/sequence/der_decode_sequence_flexi.c

@@ -53,6 +53,25 @@ static unsigned long fetch_length(const unsigned char *in, unsigned long inlen,
    return z+*data_offset;
 }
 
+static int new_element(ltc_asn1_list **l)
+{
+   /* alloc new link */
+   if (*l == NULL) {
+      *l = XCALLOC(1, sizeof(ltc_asn1_list));
+      if (*l == NULL) {
+         return CRYPT_MEM;
+      }
+   } else {
+      (*l)->next = XCALLOC(1, sizeof(ltc_asn1_list));
+      if ((*l)->next == NULL) {
+         return CRYPT_MEM;
+      }
+      (*l)->next->prev = *l;
+      *l = (*l)->next;
+   }
+   return CRYPT_OK;
+}
+
 /**
    ASN.1 DER Flexi(ble) decoder will decode arbitrary DER packets and create a linked list of the decoded elements.
    @param in      The input buffer
@@ -73,6 +92,13 @@ int der_decode_sequence_flexi(const unsigned char *in, unsigned long *inlen, ltc
    l = NULL;
    totlen = 0;
 
+   if (*inlen == 0) {
+      /* alloc new link */
+      if ((err = new_element(&l)) != CRYPT_OK) {
+         goto error;
+      }
+   }
+
    /* scan the input and and get lengths and what not */
    while (*inlen) {
       /* read the type byte */
@@ -86,20 +112,8 @@ int der_decode_sequence_flexi(const unsigned char *in, unsigned long *inlen, ltc
       }
 
       /* alloc new link */
-      if (l == NULL) {
-         l = XCALLOC(1, sizeof(*l));
-         if (l == NULL) {
-            err = CRYPT_MEM;
-            goto error;
-         }
-      } else {
-         l->next = XCALLOC(1, sizeof(*l));
-         if (l->next == NULL) {
-            err = CRYPT_MEM;
-            goto error;
-         }
-         l->next->prev = l;
-         l = l->next;
+      if ((err = new_element(&l)) != CRYPT_OK) {
+         goto error;
       }
 
       if ((type & 0x20) && (type != 0x30) && (type != 0x31)) {