ixgbe: fix potential null dereference

This patch adds a default case which goes to the next loop iteration
in the case where p is not set, preventing p from being dereferenced.

Signed-off-by: Josh Hay <joshua.a.hay@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>