net: dns: dispatcher: fix OOB array access

Validate that the file descriptor is not a negative number before
writing to the `dispatch_table` `ctx` field. Setting file descriptors
to `-1` is the standard "not in use" value, and in fact the entire array
of `fds` is set to this value in `dns_resolve_init_locked`. This
resolves memory corruption of whichever variable is unfortunate to exist
just before `dispatch_table` in memory.

Signed-off-by: Jordan Yates <jordan@embeint.com>