fix warning in md5.c

fix warning:

cc -Wall -O2 -DNDEBUG -I libuv/include -std=gnu99 -o \
        server server.c utils.c encrypt.c md5.c rc4.c \
        libuv/libuv.a -lpthread -lcrypto -lm -lrt
md5.c: In function ‘MD5Final’:
md5.c:146:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
md5.c:147:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]