Use borrowed data when exporting an image created by Image.fromarrow

[lazerg]

Resolves #9896

An image created by Image.fromarrow() borrows its data, so block and blocks are both NULL and only the row pointers in image are set. export_single_channel_array() and export_fixed_pixel_array() fall back to blocks[0].ptr when block is NULL, which dereferenced NULL and crashed on pa.array(im).

A borrowed array is only accepted when its length matches the image, so the rows are contiguous and the export can start from the first one.

[radarhere]

I've created lazerg#1 with a suggestion.

[lazerg]

Good catch, thanks. RGBX/CMYK/YCbCr/HSV route through the same export_fixed_pixel_array path as RGBA (branch is picked purely by im->bands == 1, not by mode), so the fix covers them too. Applied your refactor and ran all 10 modes locally: all pass, including the 4 new ones. Pushed as bd55c41.