base64: fix recent compilation regression on some compilers

base64: fix recent compilation regression on some compilers

This fixes a compile failure with
  "Sun C 5.8 Patch 121015-05 2007/08/01"
which returns:
  "base64.c", line 99: void function cannot return value

* lib/base64.c: Don't return the void function,
instead split to a separate return statement.