ximgproc: fix heap buffer overflow in EdgeDrawing::detectLines()

[purehol]

Root cause

detectLines() allocated its line-fitting scratch buffers x/y as
(width + height) * 8 doubles, then copied every pixel of each edge segment into
them. An edge segment is a 1-pixel-wide chain that can wind through the whole
image, so its length is bounded by width * height, not by the image perimeter.
A segment longer than the buffer overflows it at the fill loop — heap corruption;
AddressSanitizer reports a WRITE past the end at edge_drawing.cpp:1409.

Fix

Size the buffers to the longest segment, mirroring the reference implementation
ED_Lib (CihanTopal/ED_Lib@2778deb). The (size_t) cast avoids int overflow on
large images.

Verification

Added TEST_F(ximgproc_ED, detectLinesLongWindingSegment): a single serpentine
segment (~17k px, far larger than the (width+height)*8 buffer). It asserts the
oversized-segment precondition, then calls detectLines(). Under AddressSanitizer
this triggers a heap-buffer-overflow before the change and is clean after it; the
existing opencv_test_ximgproc *ED* tests still pass.

This may be the cause of #3429 (a detectLines segfault whose reproducibility
depends on heap layout, consistent with this overflow). Ordinary photographs do
not produce a segment large enough to overflow, so it could not be verified
against that reporter's specific input; the reporters have been asked to confirm.

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