externals: Add catch2 v3.2.1

Merge commit '6879e5bb1c598a9a517d7bdec8ba1a0bace3ef10' as 'externals/catch'

externals/catch/fuzzing/fuzz_XmlWriter.cpp

@@ -0,0 +1,16 @@
+//License: Boost 1.0
+//By Paul Dreik 2020
+
+#include <catch2/internal/catch_xmlwriter.hpp>
+
+#include "NullOStream.h"
+
+extern "C" int LLVMFuzzerTestOneInput(const uint8_t *Data, size_t Size) {
+
+    std::string buf(Data,Data+Size);
+    NullOStream nul;
+    Catch::XmlEncode encode(buf);
+    encode.encodeTo(nul);
+    return 0;
+}
+