9pnet_rdma: check token type before int conversion

When parsing options, make sure we have found a proper token before
doing a numeric conversion.

Without this check, the current code will end up following random
pointers that just happened to be on the stack when this function was
called, because match_token() will not touch the 'args' list unless a
valid token is found.

Signed-off-by: Simon Derr <simon.derr@bull.net>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>