xfeatures2d: fix out-of-bounds read in PCTSignaturesSQFD::computeQuadraticFormDistances

[shimat]

Parallel_computeSQFDs::operator() in pct_signatures_sqfd.cpp indexes mImageSignatures (a const std::vector<Mat>*) as mImageSignatures[i] instead of (*mImageSignatures)[i]. This happens to work for i == 0, but for i >= 1 it reads past the pointed-to vector as unallocated memory, which is undefined behavior and in practice throws a spurious Signature ID: N is empty! once imageSignatures has 2 or more elements.

Added a regression test in test_pct_signatures_sqfd.cpp that calls computeQuadraticFormDistances with 3 signatures and checks each result against a direct computeQuadraticFormDistance call.

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