[scudo] Ensure that reallocate copies everything
getUsableSize returns the actual capacity of the underlying block, which
may be larger than the size originally requested by the user. If the
user writes data into this extra space accessible via getUsableSize and
subsequently calls reallocate, the existing implementation only copies
the original requested number of bytes. This resulted in data loss for
any information stored beyond the requested size but within the usable
bounds.
