Commit 8ba0a58f introduced a compiler warning that I hit during
a run of ./autobuild.sh:

../../src/nodeinfo.c: In function 'nodeCapsInitNUMA':
../../src/nodeinfo.c:1853:43: error: 'nsiblings' may be used uninitialized in this function [-Werror=maybe-uninitialized]
         if (virCapabilitiesAddHostNUMACell(caps, n, memory,
                                           ^

Sure enough, nsiblings starts uninitialized, and is set by a call
to virNodeCapsGetSiblingInfo, but that function fails to assign
through the pointer if virNumaGetDistances fails.

* src/nodeinfo.c (nodeCapsInitNUMA): Initialize nsiblings.
Signed-off-by: NEric Blake <eblake@redhat.com>