For reference, here's the signature of babl_space_from_icc:
const Babl *babl_space_from_icc (const char       *icc_data,
                                 int               icc_length,
                                 BablIccIntent     intent,
                                 const char      **error);

... and JOCTET can be defined as either a signed or unsigned char:
#ifdef HAVE_UNSIGNED_CHAR
typedef unsigned char JOCTET;
#define GETJOCTET(value)  (value)
#else /* not HAVE_UNSIGNED_CHAR */
typedef char JOCTET;
#ifdef __CHAR_UNSIGNED__
#define GETJOCTET(value)  (value)
#else
#define GETJOCTET(value)  ((value) & 0xFF)
#endif /* __CHAR_UNSIGNED__ */
#endif /* HAVE_UNSIGNED_CHAR */