fix(koptreflow): margin calculation, memory leaks, and div-by-zero

[tachibana-shin]

• Fix copy-paste error: bottom margin used top margin index (box[1] → box[3])
• Free previous BOXA/NUMA/wrectmaps before overwriting to prevent memory leaks
• Guard division by zero when srcdpiw, srcdpih, or zoom is zero
• Add input validation in pixmap_to_bmp() for null data and invalid dimensions

Problem 1: Copy-paste error in margin calculation

In lib/koptreflow.c:110, the bottom margin was calculated using the wrong index:

martop = (int) (k2settings->dst_dpi * k2settings->dstmargins.box[1] * 2 + .5);
marbot = (int) (k2settings->dst_dpi * k2settings->dstmargins.box[1] * 2 + .5);  // BUG

Both used box[1] (top margin). The box array is indexed as: box[0]=left, box[1]=top, box[2]=right, box[3]=bottom. The bottom margin should use box[3].

Fix

marbot = (int) (k2settings->dst_dpi * k2settings->dstmargins.box[3] * 2 + .5);

Problem 2: Memory leak on reflow

When k2pdfopt_reflow_bmp() is called multiple times (e.g. reflowing different pages), the previous rboxa, nboxa, rnai, nnai BOXA/NUMA objects are overwritten without being freed. Similarly, wrectmaps entries accumulate across calls.

Fix

Free previous allocations before creating new ones:

boxaDestroy(&kctx->rboxa);
boxaDestroy(&kctx->nboxa);
numaDestroy(&kctx->rnai);
numaDestroy(&kctx->nnai);
wrectmaps_clear(&kctx->rectmaps);

Problem 3: Division by zero

In the rectmaps loop, rectmap->srcdpiw, rectmap->srcdpih, and kctx->zoom are used as divisors without checking for zero:

nlbox = boxCreate(rectmap->coords[0].x*k2settings->src_dpi/rectmap->srcdpiw/kctx->zoom + ...,
                   rectmap->coords[0].y*k2settings->src_dpi/rectmap->srcdpih/kctx->zoom + ...,
                   ...);

Fix

Guard with zero checks:

BOX* nlbox = NULL;
if (rectmap->srcdpiw > 0. && rectmap->srcdpih > 0. && kctx->zoom > 0.) {
    nlbox = boxCreate(...);
}
if (nlbox != NULL)
    boxaAddBox(nboxa, nlbox, L_INSERT);

Problem 4: Unchecked input in pixmap_to_bmp()

pixmap_to_bmp() does not validate pix_data pointer or bitmap dimensions before accessing pixel memory.

Fix

if (!pix_data || bmp->width <= 0 || bmp->height <= 0)
    return;

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This change is 

[Frenzie]

@benoit-pierre You said you're also working on something. Should these PRs merged first or after?

[benoit-pierre]

I would be great if could please hold off.