entry: guard for != NULL before unreffing the pixbuf

entry: guard for != NULL before unreffing the pixbuf

In gtk_entry_get_icon_pixbuf() we unreference the pixbuf that
_gtk_icon_helper_ensure_pixbuf() gives us back, since the function
doesn't return a reference, and by doing so we're able to return the
reference owned by the cached icon helper.
Since the icon helper method can return NULL though, if no icon
properties have been set on it, guard for != NULL before unreffing the
pixbuf, as that would cause a critical warning.

https://bugzilla.gnome.org/show_bug.cgi?id=679537