conf: Clean up virDomainNumaDefNodeDistanceParseXML

conf: Clean up virDomainNumaDefNodeDistanceParseXML

Clean up the style a bit w/r/t to not using a unary operator on an
integer value that could be zero - compare vs. zero instead.

Set the def->mem_nodes[*].distances to rdist or ldist inside the
if condition - no need to set outside since the value being set
to is what was fetched.

During cleanup, be sure to initialize the ndistances on error and
use the < 0 comparison not the unary one.