Duplicate mbedtls_asn1_find_named_data in asn1write.c to avoid dep.

This commit duplicates the public function mbedtls_asn1_find_named_data()
defined in library/asn1parse.c within library/asn1write.c in order to
avoid a dependency of the ASN.1 writing module on the ASN.1 parsing module.

The duplication is unproblematic from a semantic and an efficiency
perspective becasue it is just a short list traversal that doesn't
actually do any ASN.1 parsing.

Author: Hanno Becker

library/asn1write.c

@@ -328,14 +328,36 @@ int mbedtls_asn1_write_octet_string( unsigned char **p, unsigned char *start,
     return( (int) len );
 }
 
-mbedtls_asn1_named_data *mbedtls_asn1_store_named_data( mbedtls_asn1_named_data **head,
+
+/* This is a copy of the ASN.1 parsing function mbedtls_asn1_find_named_data(),
+ * which is replicated to avoid a dependency ASN1_WRITE_C on ASN1_PARSE_C. */
+static mbedtls_asn1_named_data *asn1_find_named_data(
+                                               mbedtls_asn1_named_data *list,
+                                               const char *oid, size_t len )
+{
+    while( list != NULL )
+    {
+        if( list->oid.len == len &&
+            memcmp( list->oid.p, oid, len ) == 0 )
+        {
+            break;
+        }
+
+        list = list->next;
+    }
+
+    return( list );
+}
+
+mbedtls_asn1_named_data *mbedtls_asn1_store_named_data(
+                                        mbedtls_asn1_named_data **head,
                                         const char *oid, size_t oid_len,
                                         const unsigned char *val,
                                         size_t val_len )
 {
     mbedtls_asn1_named_data *cur;
 
-    if( ( cur = mbedtls_asn1_find_named_data( *head, oid, oid_len ) ) == NULL )
+    if( ( cur = asn1_find_named_data( *head, oid, oid_len ) ) == NULL )
     {
         // Add new entry if not present yet based on OID
         //