dav: Prevent usage of uninitialized variable

More warnings are printed when building thanks to meson port. The mounting
loop may be breaked before is_webdav variable is defined. Let's move the
initialization to another place in order to prevent usage of uninitialized
variable and the following warning:
warning: ‘is_webdav’ may be used uninitialized in this function

https://bugzilla.gnome.org/show_bug.cgi?id=786149