mmioOpen() with the length of buffer set to 0 expects the buffer to be at least
MMIO_DEFAULTBUFFER (8192) bytes in size, and using buffer of smaller size can
lead to various problems when the buffer actually gets used.

This reverts commit dbd76575.

According to MSDN, ppSecurityDescriptor must not be NULL if any of the
SID or ACL parameters are non-NULL.  This is wrong, and native
GetSecurityInfo will happily return those pointers, leaving one unable
to free the original security descriptor.