This variable should be NULL by default and gets set only if a new reference to
a surface was acquired through _cairo_surface_snapshot_get_target(). This works,
because cairo_surface_destroy(NULL) is well-defined and doesn't do anything.

Fixes the following compiler warning:

cairo-recording-surface.c:1642:24: warning: 'free_me' may be used uninitialized
in this function [-Wmaybe-uninitialized]

Signed-off-by: Uli Schlachter <psychon@znc.in>