imgproc: fix -Wstringop-overflow false-positive in minEnclosingConvexPolygon

This commit addresses a compiler warning encountered when building with GCC 13 and C++20.
The compiler triggers a false-positive -Wstringop-overflow warning in `findKSides`.

By adding `sides.reserve(k)`, we explicitly inform the compiler about the required
memory allocation, which suppresses the warning and provides a minor optimization
by avoiding potential reallocations.

modules/imgproc/src/min_enclosing_convex_polygon.cpp

@@ -913,6 +913,7 @@ std::vector<Side> Chains::reconstructHSidedChain(int h, int i, int j)
 std::vector<Side> Chains::findKSides(int k, int i, int j)
 {
     std::vector<Side> sides;
+    sides.reserve(k); // Optimization and avoiding GCC's false positive warning (-Wstringop-overflow)
     sides.push_back({i, true});
     sides.push_back({j, true});