* image.c: Integer signedness and overflow and related fixes.

* image.c: Integer signedness and overflow and related fixes.

This is not an exhaustive set of fixes, but it's time to
record what I've got.
(lookup_pixel_color, check_image_size): Remove redundant decls.
(check_image_size): Don't assume that arbitrary EMACS_INT values
fit in 'int', or that arbitrary 'double' values fit in 'int'.
(x_alloc_image_color, x_create_x_image_and_pixmap, png_load)
(tiff_load, imagemagick_load_image):
Check for overflow in size calculations.
(x_create_x_image_and_pixmap): Remove unnecessary test for
xmalloc returning NULL; that can't happen.
(xbm_read_bitmap_data): Don't assume sizes fit into 'int'.
(xpm_color_bucket): Use better integer hashing function.
(xpm_cache_color): Don't possibly over-allocate memory.
(struct png_memory_storage, tiff_memory_source, tiff_seek_in_memory)
(gif_memory_source):
Use ptrdiff_t, not int or size_t, to record sizes.
(png_load): Don't assume values greater than 2**31 fit in 'int'.
(our_stdio_fill_input_buffer): Prefer ptrdiff_t to size_t when
either works, as we prefer signed integers.
(tiff_read_from_memory, tiff_write_from_memory):
Return tsize_t, not size_t, since that's what the TIFF API wants.
(tiff_read_from_memory): Don't fail simply because the read would
go past EOF; instead, return a short read.
(tiff_load): Omit no-longer-needed casts.
(Fimagemagick_types): Don't assume size fits into 'int'.