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/hmac_drbg.c b/library/hmac_drbg.c
index 2e4b682..295da3a 100644
--- a/library/hmac_drbg.c
+++ b/library/hmac_drbg.c
@@ -97,7 +97,7 @@
 
     md_init( &ctx->md_ctx );
 
-    if( ( ret = md_init_ctx( &ctx->md_ctx, md_info ) ) != 0 )
+    if( ( ret = md_init_ctx( &ctx->md_ctx, md_info, 1 ) ) != 0 )
         return( ret );
 
     /*
@@ -171,7 +171,7 @@
 
     md_init( &ctx->md_ctx );
 
-    if( ( ret = md_init_ctx( &ctx->md_ctx, md_info ) ) != 0 )
+    if( ( ret = md_init_ctx( &ctx->md_ctx, md_info, 1 ) ) != 0 )
         return( ret );
 
     md_size = md_get_size( md_info );