Cursor: Stop crashing when certain cursors are not found.

Not all cursor themes have all the types we have defined in 
WaylandCursor::CursorType. When that happens, the corresponding entry in 
cursors_ becomes NULL. We were then passing NULL to 
wl_cursor_image_get_buffer() and crashing.

Fix it by first checking if we have a valid wl_cursor_image to pass to 
wl_cursor_image_get_buffer().

TEST=Launch chrome or content_shell with a cursor theme that doesn't have
    "top_left" or another commonly used name, such as the default Wayland
    cursor theme.