MTP: Fix crash when called with no progress callback.
MTP: Fix crash when called with no progress callback.

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