lsws: prevent overflow in sws_init_context()

In the loop:
    for (i=0; i<dstH; i++) {
        int chrI= i*c->chrDstH / dstH;

when i*c->chrDstH > INT_MAX this leads to an integer overflow, which
results in a negative value for chrI and in out-of-buffer reads. The
overflow is avoided by forcing int64_t arithmetic by casting i to
int64_t.

Fix crash, and trac issue #72.

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>
