[core] fix base64 decode when char is unsigned (fixes #2848)
[core] fix base64 decode when char is unsigned (fixes #2848)

thx, codehero

x-ref:
  "buffer_append_base64_decode() broken on compilers where char is assumed unsigned"
  https://redmine.lighttpd.net/issues/2848

lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4