Fix invalid write in virNumaGetDistances

Fix invalid write in virNumaGetDistances

==== Invalid write of size 4
====    at 0x52E678C: virNumaGetDistances (virnuma.c:479)
====    by 0x5396890: nodeCapsInitNUMA (nodeinfo.c:1796)
====    by 0x203C2B: virQEMUCapsInit (qemu_capabilities.c:960)
====  Address 0xe10a1e0 is 0 bytes after a block of size 0 alloc'd
====    at 0x4C2A6D0: calloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
====    by 0x52A10D6: virAllocN (viralloc.c:191)
====    by 0x52E674D: virNumaGetDistances (virnuma.c:470)
====    by 0x5396890: nodeCapsInitNUMA (nodeinfo.c:1796)
====    by 0x203C2B: virQEMUCapsInit (qemu_capabilities.c:960)