fix an issue spotted by the Clang analyzer (potential malloc(0) call)
fix an issue spotted by the Clang analyzer (potential malloc(0) call)

byterun/backtrace.c:142:12: warning: Call to 'malloc' has an allocation size of 0 bytes
  events = malloc(*num_events * sizeof(struct ev_info));
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

git-svn-id: e7e61699ce1738538ffebb75374fbea0924bc775@16332 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02

eliberis/ocaml,yunxing/ocaml,yunxing/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,yunxing/ocaml,eliberis/ocaml,eliberis/ocaml,yunxing/ocaml,msprotz/ocaml,eliberis/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,msprotz/ocaml,msprotz/ocaml,yunxing/ocaml,eliberis/ocaml