gentypes: zero out structure in _dispose function

gentypes: zero out structure in _dispose function

Original the structure was memset to a poison value. That prevented
_dispose to be made idempotent. We should stop doing so.

Memseting the structure to 0 makes all pointers in structure become
NULL, which can be handled by free().

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>