Handle NULL function pointers as not supported errors
Handle NULL function pointers as not supported errors

Original git commit by Alexander Larsson <alexl@redhat.com> at 1188559995 +0200

svn path=/trunk/; revision=876

xkahn/gvfs-cmis,philipl/gvfs,cpatulea/gvfs,halfline/gvfs,gicmo/gvfs,philipl/gvfs,cpatulea/gvfs,xkahn/gvfs-cmis,halfline/gvfs,thiblahute/gvfs,gicmo/gvfs,gicmo/gvfs,halfline/gvfs,xkahn/gvfs-cmis,thiblahute/gvfs,xkahn/gvfs-cmis,gicmo/gvfs,philipl/gvfs