Fix benign case of uninitialized variable

When the "resnsend" command is throttled by the query limiter, rcode
was uninitialized. The -Wconditional-uninitialized flag detects this,
but since resNSendToAiError() doesn't read rcode when nsendAns < 0,
there was no actual bug.

Change-Id: I5a7cb2ab5d627ace5b12377025b99c380fbf0beb