sequence-point warning with gcc -Wall

[greghudson]

Building with gcc version 7.5.0 or 9.3.0 and the -Wall flag, I see several warnings of the form:

aeskey.c: In function ‘aes_decrypt_key192’:
aeskey.c:407:34: warning: operation on ‘ss[4]’ may be undefined [-Wsequence-point]
     cx->ks[v(48,(4))] = ff(ss[4] = word_in(key, 4));
aesopt.h:363:25: note: in definition of macro ‘to_byte’
 #  define to_byte(x)  ((x) & 0xff)
                         ^
aesopt.h:737:12: note: in expansion of macro ‘bval’
   ^ tab[2][bval(vf(x,2,c),rf(2,c))] \
            ^~~~
aesopt.h:737:17: note: in expansion of macro ‘vf1’
   ^ tab[2][bval(vf(x,2,c),rf(2,c))] \
                 ^~
aesopt.h:759:29: note: in expansion of macro ‘four_tables’
 #  define inv_mcol(x)       four_tables(x,t_use(i,m),vf1,rf1,0)
                             ^~~~~~~~~~~
aeskey.c:242:17: note: in expansion of macro ‘inv_mcol’
 #define ff(x)   inv_mcol(x)
                 ^~~~~~~~
aeskey.c:407:25: note: in expansion of macro ‘ff’
     cx->ks[v(48,(4))] = ff(ss[4] = word_in(key, 4));
                         ^~

four_tables() uses its first argument four times, so using it with a side-effecting expression may be the source of the complaint.

[greghudson]

Fixed by PR #55.