[CN] segfault with large global array

[peterohanley]

long a[10000000] = 0;

% cn verify big.c
zsh: segmentation fault  cn verify big.c
% cat big.c 
long a[10000000] = 0;
% cn --version
git-85e538b14-dirty [2025-02-07 16:59:43 +0000]
% lldb cn
(lldb) target create "cn"
Current executable set to '/Users/guso/.opam/default/bin/cn' (arm64).
(lldb) run verify big.c
Process 41142 launched: '/Users/guso/.opam/default/bin/cn' (arm64)
Process 41142 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x16f603ff0)
    frame #0: 0x0000000199dafed8 libsystem_malloc.dylib`_nanov2_free + 4
libsystem_malloc.dylib`_nanov2_free:
->  0x199dafed8 <+4>:  stp    x24, x23, [sp, #-0x40]!
    0x199dafedc <+8>:  stp    x22, x21, [sp, #0x10]
    0x199dafee0 <+12>: stp    x20, x19, [sp, #0x20]
    0x199dafee4 <+16>: stp    x29, x30, [sp, #0x30]
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x16f603ff0)
  * frame #0: 0x0000000199dafed8 libsystem_malloc.dylib`_nanov2_free + 4
    frame #1: 0x0000000199d8fe38 libsystem_malloc.dylib`nanov2_realloc + 360
    frame #2: 0x0000000199dbbc60 libsystem_malloc.dylib`_malloc_zone_realloc + 144
    frame #3: 0x0000000199dbc47c libsystem_malloc.dylib`_realloc + 468
    frame #4: 0x0000000102140310 libgmp.10.dylib`__gmp_default_reallocate + 32
    frame #5: 0x00000001021522d8 libgmp.10.dylib`__gmpz_realloc + 68
    frame #6: 0x000000010214fd98 libgmp.10.dylib`__gmpz_n_pow_ui + 1276
    frame #7: 0x0000000100af9dc4 cn`ml_z_pow + 336
    frame #8: 0x0000000100b298f4 cn`caml_call_gc + 352
(lldb) 

Without the initialization it's just a stack overflow, looks like #778

% cn verify big.c
cn: internal error, uncaught exception:
    Stack overflow
    Raised at Stdlib__Hashtbl.find in file "hashtbl.ml", line 543, characters 17-32
    Called from C_lexer.__ocaml_lex_initial_rec in file "parsers/c/c_lexer.mll", line 618, characters 8-31
% cat big.c
long a[10000000];