bug#26694: preview-at-point fails on very long lines with "End of buffer"

[Frank Fischer]

Hi,

consider the following example file https://pastebin.com/73W7EbCa

This file has a single very long line with one math item at the end. Do
the following:

1. generate the preview with C-c C-p C-p
2. move point to the "x" in the math block, change it to "y" and leave
point within the math block
3. execute C-c C-p C-p again

One should get the message "End of buffer" without the preview being
regenerated.

AFAICT the reason is that in this case only the snippet $x$ without the
text preceding the snippet in the line is copied to the temporary file
for generating the preview. However line 2103 in tex-buf.el (that is the
line with "(forward-char (cdr line-col))") tries to move point to the
*original* column of the snippet. This number is so large that
"(forward-char ...)" will actually reach the end of the buffer and the
error occurs.

However, I would say the the column offset is just wrong. Either
subtract the length of the preceding text (in the original buffer) from
the column number or copy the preceding text to the temp buffer as well.

I hope my description is clear, if not, please ask ;)

Best regards,
Frank