Revert back to malloc() in rolling_median_t

-   The change to `calloc()` in the `rolling_median_t` constructor has
    no effect on the `valgrind` warnings, so revert back to the original
    version of the code.

contrib/epee/include/rolling_median.h

@@ -131,7 +131,7 @@ struct rolling_median_t
   rolling_median_t(size_t N): N(N)
   {
     int size = N * (sizeof(Item) + sizeof(int) * 2);
-    data = (Item*)calloc(size, 1);
+    data = (Item*)malloc(size);
     pos = (int*) (data + N);
     heap = pos + N + (N / 2); //points to middle of storage.
     clear();