Fix ProofGeneral/PG#413 further

The previous code raised some bytecomp errors when loading a ~/.emacs
containing a bootstrapped use-package proof-general config., such as:

```
(require 'package)
(let* ((no-ssl (and (memq system-type '(windows-nt ms-dos))
                    (not (gnutls-available-p))))
       (proto (if no-ssl "http" "https")))
  (add-to-list 'package-archives
               (cons "melpa" (concat proto "://melpa.org/packages/")) t))
(package-initialize)

;; Bootstrap use-package
(unless (package-installed-p 'use-package)
  (package-refresh-contents)
  (package-install 'use-package))

(eval-when-compile
  (require 'use-package))

(use-package proof-general
  :ensure t
  :mode ("\\.v\\'" . coq-mode))
```

1 files changed, 3 insertions, 1 deletions

diff --git a/proof-general.el b/proof-general.el
index a2f8c6fa..c7fd639e 100644
--- a/proof-general.el
+++ b/proof-general.el
@@ -73,7 +73,9 @@
            ;; them for the compilation to succeed for me.  --Stef
            "isar" "lego" "twelf" "obsolete/plastic"))
         (root (file-name-directory
-               (or load-file-name byte-compile-current-file buffer-file-name))))
+               (or load-file-name
+		   (bound-and-true-p byte-compile-current-file)
+		   buffer-file-name))))
     (dolist (dir byte-compile-directories)
       (add-to-list 'load-path (expand-file-name dir root)))))