Fix rare SIGSEGV when transferring large replicas.
Fix a theoretical integer overflow. 

Patches from here:
https://caml.inria.fr/mantis/view.php?id=7431#c17026
and here:
https://caml.inria.fr/mantis/view.php?id=7431#c16962

Related issue reports:
https://github.com/bcpierce00/unison/issues/48
https://caml.inria.fr/mantis/view.php?id=7431
https://bugzilla.redhat.com/show_bug.cgi?id=1401759

Thanks to Alex Markley and OCaml developers
--- a/bytearray_stubs.c	Tue Jan 17 08:41:00 2017
+++ b/bytearray_stubs.c	Tue Jan 17 08:41:21 2017
@@ -5,6 +5,7 @@
 
 #include "caml/intext.h"
 #include "caml/bigarray.h"
+#include "caml/memory.h"
 
 CAMLprim value ml_marshal_to_bigarray(value v, value flags)
 {
@@ -21,15 +22,18 @@ CAMLprim value ml_marshal_to_bigarray(value v, value f
 
 CAMLprim value ml_unmarshal_from_bigarray(value b, value ofs)
 {
+  CAMLparam1(b); /* Holds [b] live until unmarshalling completes. */
+  value result;
   struct caml_bigarray *b_arr = Bigarray_val(b);
-  return input_value_from_block (Array_data (b_arr, ofs),
+  result = input_value_from_block (Array_data (b_arr, ofs),
                                  b_arr->dim[0] - Long_val(ofs));
+  CAMLreturn(result);
 }
 
 CAMLprim value ml_blit_string_to_bigarray
 (value s, value i, value a, value j, value l)
 {
-  char *src = String_val(s) + Int_val(i);
+  char *src = String_val(s) + Long_val(i);
   char *dest = Array_data(Bigarray_val(a), j);
   memcpy(dest, src, Long_val(l));
   return Val_unit;