[Minor] Escape \uFFFD in the sources

src/libserver/html/html_entities.cxx

@@ -2605,10 +2605,10 @@ TEST_SUITE("html entities") {
 				{"FOO&#ZOO", "FOO&#ZOO"},
 				{"FOO&#xBAR", "FOOºR"},
 				{"FOO&#x41BAR", "FOO䆺R"},
-				{"FOO�ZOO", "FOO�ZOO"},
+				{"FOO�ZOO", "FOO\uFFFDZOO"},
 				{"FOOZOO", "FOO\u0081ZOO"},
-				{"FOO�ZOO", "FOO�ZOO"},
-				{"FOO�ZOO", "FOO�ZOO"},
+				{"FOO�ZOO", "FOO\uFFFDZOO"},
+				{"FOO�ZOO", "FOO\uFFFDZOO"},
 				{"ZZ&pound_id=23", "ZZ£_id=23"},
 				{"ZZ&prod_id=23", "ZZ&prod_id=23"},
 				{"ZZ&gt", "ZZ>"},

src/libutil/cxx/utf8_util.cxx

@@ -225,7 +225,7 @@ TEST_CASE("utf8 normalise") {
 			{"13\u200C_\u0020\u0308\u0301\u038e\u03ab\u200D", "13_ ̈́ΎΫ",
 					RSPAMD_UNICODE_NORM_UNNORMAL|RSPAMD_UNICODE_NORM_ZERO_SPACES},
 			/* Buffer overflow case */
-			{"u\xC2\xC2\xC2\xC2\xC2\xC2""abcdef""abcdef", "u������",
+			{"u\xC2\xC2\xC2\xC2\xC2\xC2""abcdef""abcdef", "u\uFFFD\uFFFD\uFFFD\uFFFD\uFFFD\uFFFD",
 					RSPAMD_UNICODE_NORM_UNNORMAL|RSPAMD_UNICODE_NORM_ERROR},
 	};