Fixed nil pointer dereference in url validation code that
caused kubeamd panic:

  panic: runtime error: invalid memory address or nil pointer dereference
  [signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0xa7930c]

  goroutine 1 [running]:
  kubeadm/validation.ValidateURLs(0x40000bafe0, 0x2, 0x2, 0x1, 0x40002967b0, 0x0, 0x40002967b0, 0xf302a0)
    kubeadm/validation/validation.go:324 +0xcc
  kubeadm/validation.ValidateEtcd(0x400000b490, 0x4000296720, 0x0, 0x0, 0x0)
    kubeadm/validation/validation.go:291 +0x1f0
      ...

Fixes: kubernetes/kubeadm#1419
Signed-off-by: Ed Bartosh <eduard.bartosh@intel.com>