xen/gunzip: Allow perform_gunzip() to be called multiple times

xen/gunzip: Allow perform_gunzip() to be called multiple times

Currently perform_gunzip() can only be called once because the
the internal state (e.g allocate) is not fully re-initialized.

This works fine if you are only booting dom0. But this will break when
booting multiple using the dom0less that uses compressed kernel images.

This can be resolved by re-initializing bytes_out, malloc_ptr,
malloc_count every time perform_gunzip() is called.

Note the latter is only re-initialized for hardening purpose as there is
no guarantee that every malloc() are followed by free() (It should in
theory!).

Take the opportunity to check the return of alloc_heap_pages() to return
an error rather than dereferencing a NULL pointer later on failure.

Reported-by: Charles Chiou <cchiou@ambarella.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>