udisks2: Initialize local variables to NULL
udisks2: Initialize local variables to NULL

We need to set initial value to local variables we're about to use
later in the code.

Found the volume monitor segfaulting on stable udisks-2.0.0

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