load_ini_buffer: allocate ini_length + 1 bytes (fixes OOB write)

[1820893135-pixel]

Fixes the heap-buffer-overflow reported in #20.

strip_ini_cache() documents (and does) a mandatory one-byte write:

/* src/confini.c:2621 — first statement of strip_ini_cache() */
ini_source[ini_length] = '\0';

so its ini_source argument needs ini_length + 1 usable bytes. The bundled helper examples/utilities/load_ini_buffer.h violated that contract:

char * const ini_cache = malloc(ini_length);   /* only ini_length bytes */
memcpy(ini_cache, ini_buffer, ini_length);
const int retval = strip_ini_cache(ini_cache, ini_length, ...);

Every non-empty input therefore wrote one NUL byte past the allocation. Other in-tree callers are fine — examples/topics/strip_ini_cache.c uses strndup() and the two doc examples in src/confini.c use malloc(file_size + 1) — this helper was the only one missing the extra byte.

Fix: allocate ini_length + 1 bytes to match the documented contract.

Verification

ASan reproducer using the bundled header itself:

#include <confini.h>
#include "load_ini_buffer.h"
static int cb(IniDispatch *d, void *v) { (void)d; (void)v; return 0; }
int main(void) {
    const char *input = "[S]\nkey = value\n";
    return load_ini_buffer(input, strlen(input), INI_DEFAULT_FORMAT, NULL, cb, NULL);
}

clang -fsanitize=address -I src -I examples/utilities test.c src/confini.c -o test

Before: aborts with

ERROR: AddressSanitizer: heap-buffer-overflow ... WRITE of size 1
    #0 strip_ini_cache src/confini.c:2621:25
    #1 load_ini_buffer  examples/utilities/load_ini_buffer.h:43:22

After: exits 0 with no sanitizer report.

I also checked the UTF-8 BOM probe at src/confini.c:2636-2640 (ini_source[0..2]): it is not separately exploitable, because the mandatory ini_source[ini_length] = '\0' always terminates that three-byte chain first (ini_source[2] is only read when ini_length >= 3, in which case it is in bounds). So the allocation is the only change needed.

Fixes #20.