readonly SvMemoryStream shouldn't allow resize

Change-Id: I4cd1969cc961d48f904460371ab0a0f960bdea1e
Reviewed-on: https://gerrit.libreoffice.org/34832
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

tools/source/stream/stream.cxx

@@ -1830,7 +1830,7 @@ bool SvMemoryStream::AllocateMemory( std::size_t nNewSize )
 // (using Bozo algorithm)
 bool SvMemoryStream::ReAllocateMemory( long nDiff )
 {
-    if (!bOwnsData)
+    if (!m_isWritable || !bOwnsData)
         return false;
 
     bool bRetVal    = false;
@@ -1916,6 +1916,12 @@ void* SvMemoryStream::SwitchBuffer()
 
 void SvMemoryStream::SetSize(sal_uInt64 const nNewSize)
 {
+    if (!m_isWritable)
+    {
+        SetError(SVSTREAM_INVALID_HANDLE);
+        return;
+    }
+
     long nDiff = (long)nNewSize - (long)nSize;
     ReAllocateMemory( nDiff );
 }