md5: code shrink; and use 64-byte temp buf, not 128-byte.

md5: code shrink; and use 64-byte temp buf, not 128-byte.

function                                             old     new   delta
md5_hash                                             111     108      -3
md5_end                                              129     125      -4
md5_hash_block                                       459     454      -5
filter_rename_config                                 250     244      -6
md5_crypt                                            587     578      -9
popmaildir_main                                      828     816     -12
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/6 up/down: 0/-39)             Total: -39 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>