texttopdf: range-check the column-width computation

[ManasBagul23]

Problem

write_prolog() derives ColumnWidth from the "columns" option
(doc->PageColumns), which only has a lower-bound check (>= 1) — there is
no upper bound. The computation itself:

doc->ColumnWidth = (doc->SizeColumns - doc->ColumnGutter *
                    (doc->PageColumns - 1)) /
                    doc->PageColumns;

is done entirely in plain int arithmetic. A large PageColumns value
(e.g. columns=429496747) overflows the ColumnGutter * (PageColumns - 1)
product (and/or the final division), wrapping what should be a deeply
negative/rejected result into a small positive ColumnWidth instead.

That bogus ColumnWidth then corrupts the page-column write-site math in
cfFilterTextToPDF(): i = column + page_column * (ColumnWidth + ColumnGutter) is used to index doc->Page[line][i] directly with no
further bound check, and page_column is only bounds-checked against the
(also wrong, since it mirrors the same user input) doc->PageColumns. With
ColumnWidth wrongly small, page_column can climb far past what
doc->SizeColumns cells were actually allocated for, producing a
heap-buffer-overflow write.

Fixes #195

Fix

Do the gutter product and the resulting column width in long long first,
and reject the page (matching write_prolog()'s existing error convention
— log and return 1, already used a few lines above for a bad charset file)
if the real result doesn't fit between 1 and doc->SizeColumns, instead of
silently using a wrapped value.

Verified

No C build toolchain available locally to compile-check. Verified by
tracing doc->ColumnWidth's only two downstream uses that matter for
allocation safety: the page_column >= doc->PageColumns page-wrap check
(which the corrupted PageColumns value itself doesn't protect) and the
direct doc->Page[line][i] write in cfFilterTextToPDF(), confirming the
invariant page_column * (ColumnWidth + ColumnGutter) + column < SizeColumns only holds when ColumnWidth is computed without overflow.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.