Fix aliasing bug in mp_lshift_fixed_into.

I had not considered what would happen if the input and output mp_ints
aliased each other. What happens is that because I wrote the output
starting from the low end, input words would get corrupted before I'd
finished with them. Easily fixed by reversing the order of the loop.

mpint.c

@@ -1033,7 +1033,7 @@ void mp_lshift_fixed_into(mp_int *r, mp_int *a, size_t bits)
     size_t words = bits / BIGNUM_INT_BITS;
     size_t bitoff = bits % BIGNUM_INT_BITS;
 
-    for (size_t i = 0; i < r->nw; i++) {
+    for (size_t i = r->nw; i-- > 0 ;) {
         if (i < words) {
             r->w[i] = 0;
         } else {