* image.c: Improve checking for integer overflow.

* image.c: Improve checking for integer overflow.

(check_image_size): Assume that f is nonnull, since
it is always nonnull in practice.  This is one less thing to
worry about when checking for integer overflow later.
(x_check_image_size): New function, which checks for integer
overflow issues inside X.
(x_create_x_image_and_pixmap, xbm_read_bitmap_data): Use it.
This removes the need for a memory_full check.
(xbm_image_p): Rewrite to avoid integer multiplication overflow.
(Create_Pixmap_From_Bitmap_Data, xbm_load): Use x_check_image_size.
(xbm_read_bitmap_data): Change locals back to 'int', since
their values must fit in 'int'.
(xpm_load_image, png_load, tiff_load):
Invoke x_create_x_image_and_pixmap earlier,
to avoid much needless work if the image is too large.
(tiff_load): Treat overly large images as if
x_create_x_image_and_pixmap failed, not as malloc failures.
(gs_load): Use x_check_image_size.