tdf#67538 XTypeDetection::queryTypeByDescriptor poor performance, part3

Size the OUStringBuffer in OrcusFormatDetect::detect better at
construction, to avoid re-alloc.

This is specifically fixing the performance of queryTypeByDescriptor
when called from a basic macro on a local test file.

This takes my test macro from 9s to 8s

Change-Id: Id4c2bf386de172b1deed98efece8c1f040c599a5
Reviewed-on: https://gerrit.libreoffice.org/73376
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

sc/source/filter/orcus/filterdetect.cxx

@@ -68,11 +68,11 @@ OUString OrcusFormatDetect::detect(css::uno::Sequence<css::beans::PropertyValue>
         return OUString();
 
     css::uno::Reference<css::io::XInputStream> xInputStream(aMediaDescriptor[utl::MediaDescriptor::PROP_INPUTSTREAM()], css::uno::UNO_QUERY );
+    OStringBuffer aContent(xInputStream->available());
 
     static const sal_Int32 nBytes = 4096;
     css::uno::Sequence<sal_Int8> aSeq(nBytes);
     bool bEnd = false;
-    OStringBuffer aContent;
     while(!bEnd)
     {
         sal_Int32 nReadBytes = xInputStream->readBytes(aSeq, nBytes);