Somewhere between OCaml 4.01 and OCaml 4.04, <caml/memory.h> changed
the way that CAMLlocalX() values on the stack are initialized. In
OCaml 4.01:
#define CAMLlocal1(x) \
value x = 0; \
CAMLxparam1 (x)
but in OCaml 4.04:
#define CAMLlocal1(x) \
value x = Val_unit; \
CAMLxparam1 (x)
The code in mllib/visit-c.c assumed that the value would be
initialized to Val_unit, and used a check (exn != Val_unit) to see if
the value had been updated. This failed badly (with a segfault) on
OCaml 4.01 in Debian 8.
Resolve this by always initializing CAMLlocalX() values before use.
57d17ca301