Make hmac_ctx optional

Note from future self: actually md_init_ctx will be re-introduced with the
same signature later, and a new function with the additional argument will be
added.
diff --git a/library/pkcs12.c b/library/pkcs12.c
index 66b29c1..1f442e6 100644
--- a/library/pkcs12.c
+++ b/library/pkcs12.c
@@ -268,7 +268,7 @@
 
     md_init( &md_ctx );
 
-    if( ( ret = md_init_ctx( &md_ctx, md_info ) ) != 0 )
+    if( ( ret = md_init_ctx( &md_ctx, md_info, 0 ) ) != 0 )
         return( ret );
     hlen = md_get_size( md_info );