c/openmp_matrix: initialize loop variables for clang 9.0
c/openmp_matrix: initialize loop variables for clang 9.0

clang 9.0 on Sabayon issues warning about the loop index variables:

    openmp_matrix.c:59:20: error: variable 'i' is uninitialized when
    used here [-Werror,-Wuninitialized]
                matrix[i * size + j] = value;
                       ^
    openmp_matrix.c:54:13: note: initialize the variable 'i' to silence this warning
        size_t i, j;
                ^
                 = 0
    openmp_matrix.c:73:21: error: variable 'i' is uninitialized when
    used here [-Werror,-Wuninitialized]
                vectors[i * dim + j] = ((double)i) + ((double)j + 1) / 100.;
                        ^
    openmp_matrix.c:69:13: note: initialize the variable 'i' to silence this warning
        size_t i, j;
                ^
                 = 0
    openmp_matrix.c:87:23: error: variable 'i_j' is uninitialized when
    used here [-Werror,-Wuninitialized]
            sum += matrix[i_j];
                          ^~~
    openmp_matrix.c:83:15: note: initialize the variable 'i_j' to silence this warning
        size_t i_j;
                  ^
                   = 0
    openmp_matrix.c:104:20: error: variable 'i_j' is uninitialized when
    used here [-Werror,-Wuninitialized]
            size_t i = i_j / size;
                       ^~~
    openmp_matrix.c:101:15: note: initialize the variable 'i_j' to silence this warning
        size_t i_j;
                  ^
                   = 0
    openmp_matrix.c:123:16: error: variable 'i' is uninitialized when
    used here [-Werror,-Wuninitialized]
            matrix[i * size + i] = 0;
                   ^
    openmp_matrix.c:119:13: note: initialize the variable 'i' to silence this warning
        size_t i;
                ^
                 = 0

fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared,fishilico/shared