reserve_coo should zero-initialize all memory

[rileyjmurray]

This is a minor issue. Debugging would be easier if we zero-initialized memory here:

RandBLAS/RandBLAS/sparse_data/coo_matrix.hh

Lines 285 to 287 in dea0ae0

	M.vals = new T[nnz];
	M.rows = new sint_t[nnz];
	M.cols = new sint_t[nnz];