Adding a setting doesn't check for existing trailing comma

What steps will reproduce the problem?
1. vim -u NONE -U NONE
2. :set wildignore=*.png,
3. :set wildignore+=*jpg
4. :set wildignore?
outputs wildignore=*.png,,*jpg

- What is the expected output? What do you see instead?
wildignore=*.png,*jpg

- What version of the product are you using? On what operating system?
GVim 7.4 (2013 Aug 10, compiled that ) on Windows 7

- Please provide any additional information below.
having a broken wildignore breaks tags (I could make a ticket for this, if 
you'd like):

0. $ctags -R
1. vim -u NONE -U NONE
3. :set wildignore=*.png
4. :e a_file.py
5. :tag a_method "this works
6. :set wildignore=*.png,,*.jpg
7. :tag a_method "this throws a missing tags file error

Original issue reported on code.google.com by tankorsm...@gmail.com on 18 May 2015 at 6:40

weicj / vim

Adding a setting doesn't check for existing trailing comma #365