Fix: OOM vulnerability cause by is_valid_index

An OOM vulnerability exists in the json_pointer_set function (and related functions).
See issue #916 for more details.

To fix that, added a sanity check in the is_valid_index function to limit the maximum value of a parsed array index.
Provided a configurable macro for modification.

Signed-off-by: lone <lonechan314@qq.com>

json_pointer.c

@@ -79,6 +79,16 @@ static int is_valid_index(const char *path, size_t *idx)
 	// but ULLONG_MAX will be longer than any array length so that's ok.
 	*idx = strtoull(path, NULL, 10);
 
+	// Check against a maximum to prevent excessive memory allocations.
+	// An extremely large index, even if it doesn't overflow size_t,
+	// will cause a huge memory allocation request via realloc,
+	// leading to an OOM.
+	if (*idx > JSON_C_POINTER_MAX_ARRAY_IDX)
+	{
+    	errno = EINVAL;
+    	return 0;
+	}
+
 	return 1;
 }