Unit test for new error condition

In response to Tony's review in GH Perl#23301

Author: jkeenan

lib/Unicode/UCD.t

@@ -834,6 +834,13 @@ is($ret_len, 5, "... and the returned length is 5");
 ok(! defined num("98765\N{FULLWIDTH DIGIT FOUR}", \$ret_len),
    'Verify num("98765\N{FULLWIDTH DIGIT FOUR}") isnt defined');
 is($ret_len, 5, "... but the returned length is 5");
+{
+    local $@;
+    my $ret_len = [ 5 ];
+    eval { num("98765", \$ret_len); };
+    like($@, qr/::num: second parameter must be a scalar reference/,
+        "num: Incorrect type for second parameter; must be scalar ref");
+}
 my $tai_lue_2;
 if ($v_unicode_version ge v4.1.0) {
     my $tai_lue_1 = charnames::string_vianame("NEW TAI LUE DIGIT ONE");