Revert "snapshot: Allow NULL to virDomainSnapshotObjGetDef"

This reverts commit 6b90a8473875eb34bae27856857cf6561e079089.

It turns out gcc -O2 is not happy with it, complaining:

/home/pipo/libvirt/src/qemu/qemu_driver.c: In function 'qemuDomainSnapshotCreateXML':
/home/pipo/libvirt/src/qemu/qemu_driver.c:15389:26: error: potential null pointer dereference [-Werror=null-dereference]
bool memory = snapdef->memory == VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL;
~~~~~~~^~~~~~~~
/home/pipo/libvirt/src/qemu/qemu_driver.c:15389:26: error: potential null pointer dereference [-Werror=null-dereference]
In file included from /home/pipo/libvirt/src/util/virbuffer.h:27,
from /home/pipo/libvirt/src/conf/capabilities.h:27,
from /home/pipo/libvirt/src/conf/domain_conf.h:32,
from /home/pipo/libvirt/src/qemu/qemu_agent.h:26,
from /home/pipo/libvirt/src/qemu/qemu_driver.c:40:
/home/pipo/libvirt/src/util/viralloc.h:125:34: error: potential null pointer dereference [-Werror=null-dereference]
# define VIR_ALLOC_N(ptr, count) virAllocN(&(ptr), sizeof(*(ptr)), (count), true, \
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
VIR_FROM_THIS, __FILE__, __FUNCTION__, __LINE__)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/pipo/libvirt/src/qemu/qemu_driver.c:15103:9: note: in expansion of macro 'VIR_ALLOC_N'
if (VIR_ALLOC_N(ret, snapdef->ndisks) < 0)
^~~~~~~~~~~
/home/pipo/libvirt/src/qemu/qemu_driver.c:15798:45: error: null pointer dereference [-Werror=null-dereference]
virDomainSnapshotObjGetDef(snap)->memory == VIR_DOMAIN_SNAPSHOT_LOCATION_EXTERNAL) {
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~

As the patch simplified one or two callers at the risk of making
many other callers now candidates to trigger aggressive compiler
warnings, it isn't worth it.

Signed-off-by: Eric Blake <eblake@redhat.com>