Verify that target buffer exists before switching.

Have been getting a lot of errors when installing or upgrading packatges, this should fix it i guess.

```
Debugger entered--Lisp error: (error "Selecting deleted buffer")
  git-gutter:process-diff-output(#<process git-gutter>)
  #[514 "\305!\306=\205+\307\310!\311\302!\203&r\302q\210\312!\210\f\203#\313\301!\210\314)\315\300!\262\207" [#<killed buffer> "/home/a00001/.emacs.d/packages/24.3.91/log4e-20130514.912/log4e-autoloads.el" #<killed buffer> git-gutter:enabled git-gutter:has-indirect-buffers process-status exit nil git-gutter:process-diff-output buffer-live-p git-gutter:update-diffinfo git-gutter:update-indirect-buffers t kill-buffer] 5 "\n\n(fn PROC EVENT)"](#<process git-gutter> "finished\n")
  accept-process-output(#<process melpa.milkbox.net>)
  #[0 "\302\303\300\242!)\207" [(#<process melpa.milkbox.net>) inhibit-quit nil accept-process-output] 2 "\n\n(fn)"]()
  funcall(#[0 "\302\303\300\242!)\207" [(#<process melpa.milkbox.net>) inhibit-quit nil accept-process-output] 2 "\n\n(fn)"])
  url-retrieve-synchronously("http://melpa.milkbox.net/packages/owdriver-20140527.1039.el")
  url-insert-file-contents("http://melpa.milkbox.net/packages/owdriver-20140527.1039.el")
  package-install-from-archive([cl-struct-package-desc owdriver (20140527 1039) "Quickly perform various actions on other windows" ((smartrep (0 0 3)) (log4e (0 2 0)) (yaxception (0 2 0))) single "melpa" nil ((:url . "https://github.com/aki2o/owdriver") (:keywords "convenience")) nil])
  mapc(package-install-from-archive ([cl-struct-package-desc log4e (20130514 912) "provide logging framework for elisp" nil single "melpa" nil ((:url . "https://github.com/aki2o/log4e") (:keywords "log")) nil] [cl-struct-package-desc owdriver (20140527 1039) "Quickly perform various actions on other windows" ((smartrep (0 0 3)) (log4e (0 2 0)) (yaxception (0 2 0))) single "melpa" nil ((:url . "https://github.com/aki2o/owdriver") (:keywords "convenience")) nil]))
  package-download-transaction(([cl-struct-package-desc log4e (20130514 912) "provide logging framework for elisp" nil single "melpa" nil ((:url . "https://github.com/aki2o/log4e") (:keywords "log")) nil] [cl-struct-package-desc owdriver (20140527 1039) "Quickly perform various actions on other windows" ((smartrep (0 0 3)) (log4e (0 2 0)) (yaxception (0 2 0))) single "melpa" nil ((:url . "https://github.com/aki2o/owdriver") (:keywords "convenience")) nil]))
  package-install([cl-struct-package-desc owdriver (20140527 1039) "Quickly perform various actions on other windows" ((smartrep (0 0 3)) (log4e (0 2 0)) (yaxception (0 2 0))) single "melpa" nil ((:url . "https://github.com/aki2o/owdriver") (:keywords "convenience")) nil])
  mapc(package-install ([cl-struct-package-desc owdriver (20140527 1039) "Quickly perform various actions on other windows" ((smartrep (0 0 3)) (log4e (0 2 0)) (yaxception (0 2 0))) single "melpa" nil ((:url . "https://github.com/aki2o/owdriver") (:keywords "convenience")) nil]))
  package-menu-execute()
  call-interactively(package-menu-execute nil nil)
  command-execute(package-menu-execute)
```