fuzzing: Add simple fuzz test for g_string_replace()

Now that the implementation of it is significantly more complex,
involving pointer arithmetic, it should probably be fuzzed. It’s not an
API which is obviously used to handle untrusted input, but some users of
GLib might do so.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

fuzzing/meson.build

@@ -30,6 +30,7 @@ fuzz_targets = [
   'fuzz_network_address_parse_uri',
   'fuzz_paths',
   'fuzz_resolver',
+  'fuzz_string',
   'fuzz_uri_escape',
   'fuzz_uri_parse',
   'fuzz_uri_parse_params',