Both valgrind and drmemory complain about this in
SdExportTest::testSwappedOutImageExport() via
SfxOleThumbnailProperty::ImplSave().

 Syscall param pwrite64(buf) points to uninitialised byte(s)

 UNINITIALIZED READ: reading 0x0455b1b4-0x0455b1c8 20 byte(s) within...

It appears that the stream writes out everything up to the seek position
anyway (otherwise the size check wouldn't work, with sparse files)
so make sure it's all zeroed.

Also fix SvMemoryStream::ReAllocateMemory() to zero it.

Change-Id: Id86dfa65ef6f7d1bba4810f121e01473c5fcf4c7