Skip empty URLs when inlining images

Markdown links like `[foo]()`, while basically invalid, do not usually
cause too much trouble, or at least shouldn't cause problems in
unrelated code. It so happens that if a document happens to have such
a link, `markdown-toggle-inline-images` will crash with:

    insert-file-contents-literally: not a regular file: /tmp/default-directory-path/

This patch fixes the problem by checking if the `file` path extracted
from the regex is empty before proceeding with treating it like an image.

CHANGES.md

@@ -101,7 +101,8 @@
     -   Fix precedence of inline code over inline links.
     -   Improve error reporting for `markdown` and `markdown-open`.
         ([GH-291][])
-    -   Fix M-RET binding for terminals ([GH-317][])
+    -   Fix M-RET binding for terminals. ([GH-317][])
+    -   Do not fail displaying inline images on empty links. ([GH-320][])
 
   [gh-171]: https://github.com/jrblevin/markdown-mode/issues/171
   [gh-216]: https://github.com/jrblevin/markdown-mode/issues/216
@@ -143,6 +144,7 @@
   [gh-296]: https://github.com/jrblevin/markdown-mode/issues/296
   [gh-303]: https://github.com/jrblevin/markdown-mode/pull/303
   [gh-317]: https://github.com/jrblevin/markdown-mode/pull/317
+  [gh-320]: https://github.com/jrblevin/markdown-mode/pull/320
 
 # Markdown Mode 2.3
 

markdown-mode.el

@@ -8376,7 +8376,8 @@ or \\[markdown-toggle-inline-images]."
         (let ((start (match-beginning 0))
               (end (match-end 0))
               (file (match-string-no-properties 6)))
-          (when (file-exists-p file)
+          (when (and (not (zerop (length file)))
+		     (file-exists-p file))
             (let* ((abspath (if (file-name-absolute-p file)
                                 file
                               (concat default-directory file)))