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/tests/suites/test_suite_pkcs5.function b/tests/suites/test_suite_pkcs5.function
index 6074e04..4240698 100644
--- a/tests/suites/test_suite_pkcs5.function
+++ b/tests/suites/test_suite_pkcs5.function
@@ -36,7 +36,7 @@
     TEST_ASSERT( info != NULL );
     if( info == NULL )
         return;
-    TEST_ASSERT( md_init_ctx( &ctx, info ) == 0 );
+    TEST_ASSERT( md_init_ctx( &ctx, info, 1 ) == 0 );
     TEST_ASSERT( pkcs5_pbkdf2_hmac( &ctx, pw_str, pw_len, salt_str, salt_len,
                                      it_cnt, key_len, key ) == 0 );