rndc: sync ISC_R_CANCELED handling in callbacks

rndc: sync ISC_R_CANCELED handling in callbacks

rndc_recvdone() is not treating the ISC_R_CANCELED result code as a
request to stop data processing, which may cause a crash when trying to
dereference ccmsg->buffer.  Fix by ensuring ISC_R_CANCELED results in an
early exit from rndc_recvdone().

Make sure the logic for handling ISC_R_CANCELED in rndc_recvnonce()
matches the one present in rndc_recvdone() to ensure consistent behavior
between these two sibling functions.