basenc: avoid undefined behaviour in z85 processing

basenc: avoid undefined behaviour in z85 processing

* src/basenc.c (z85_decode_ctx_init): Ensure we're working
with unsigned, as otherwise ubsan triggers with:
  src/basenc.c:767:18: runtime error: signed integer overflow:
  43 * 52200625 cannot be represented in type 'int'
(z85_encode): Likewise to avoid the usban error:
  src/basenc.c:630:26: runtime error:
  left shift of 134 by 24 places cannot be represented in type 'int'