Protect MemBlob::append() against raw-space writes

Protect MemBlob::append() against raw-space writes

There is no guarantee that the 'unused' area of MemBlob is actually
unused. For example if a read buffer was being filled into the
rawSpace() of a SBuf or MemBlob it will overlap with this empty area
until a read call updates the related size state in MemBlob/SBuf.

For these cases we must use memmove() which guarantees no buffer
corruption will take place on memory overlaps.