Check gsize against G_MAXSIZE not ULLONG_MAX

Check gsize against G_MAXSIZE not ULLONG_MAX

/builds/cgzones/rspamd/debian/output/source_dir/src/libserver/http/http_message.c: In function 'rspamd_http_message_set_body':
/builds/cgzones/rspamd/debian/output/source_dir/src/libserver/http/http_message.c:201:23: warning: comparison is always true due to limited range of data type [-Wtype-limits]
  201 |   if (len != 0 && len != ULLONG_MAX) {
      |                       ^~
/builds/cgzones/rspamd/debian/output/source_dir/src/libserver/http/http_message.c:230:23: warning: comparison is always true due to limited range of data type [-Wtype-limits]
  230 |   if (len != 0 && len != ULLONG_MAX) {
      |                       ^~