base64: use a less complicated validity check in the decode loop
base64: use a less complicated validity check in the decode loop

Checking each of the four input chars for validity generates horrible
machine code with many comparisons and conditional jumps.  We can reduce
this to just one comparison and one conditional jump by making assumptions
about what invalid inputs look like (i.e., 0xff).

Suggested by Timo Sirainen.

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc