Fix Coverity-reported ARRAY_VS_SINGLETON error.

Fix Coverity-reported ARRAY_VS_SINGLETON error.

As it turned out, this wasn't a huge deal. We were calling
ast_app_parse_options() for a set of options of which none
took arguments. The proper thing to do for this case is to
pass NULL for the "args" parameter here. We were instead passing
a seemingly-randomly chosen char * from the function. While this
would never get written to, you can rest assured things would
have gotten bad had new options (which took arguments) been added
to func_volume.

(closes issue ASTERISK-19656)
........

Merged revisions 364899 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@364900 65c4cc65-6c06-0410-ace0-fbb531ad65f3