Cleaning up pgcrypto/crypt-des.c to avoid compiler warnings

For some time now, buildfarm member serinus has been complaining
about pgcrypto/crypt-des.c:

[1601/2408] /usr/bin/ccache /usr/lib/gcc-snapshot/bin/gcc -Icontrib/pgcrypto/pgcrypto.so.p -Isrc/include -I../pgsql/src/include -I/usr/include/libxml2 -fdiagnostics-color=never -D_FILE_OFFSET_BITS=64 -Wall -Winvalid-pch -O2 -g -fno-strict-aliasing -fwrapv -fexcess-precision=standard -fpch-deps -D_GNU_SOURCE -Wpointer-arith -Werror=vla -Wmissing-format-attribute -Wcast-function-type -Wshadow=compatible-local -Wformat-security -Wmissing-prototypes -Wold-style-declaration -Wold-style-definition -Wstrict-prototypes -Wimplicit-fallthrough=5 -Wdeclaration-after-statement -Wmissing-variable-declarations -Wno-format-truncation -Wno-stringop-truncation -O1 -ggdb -g3 -fno-omit-frame-pointer -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wno-missing-field-initializers -O3 -Wno-unterminated-string-initialization -Wno-clobbered -fPIC -isystem /usr/include/mit-krb5 -isystem /usr/include/mit-krb5 -pthread -fvisibility=hidden -MD -MQ contrib/pgcrypto/pgcrypto.so.p/crypt-des.c.o -MF contrib/pgcrypto/pgcrypto.so.p/crypt-des.c.o.d -o contrib/pgcrypto/pgcrypto.so.p/crypt-des.c.o -c ../pgsql/contrib/pgcrypto/crypt-des.c
../pgsql/contrib/pgcrypto/crypt-des.c: In function 'px_crypt_des':
../pgsql/contrib/pgcrypto/crypt-des.c:675:22: warning: writing 8 bytes into a region of size 7 [-Wstringop-overflow=]
675 | *q++ = *key << 1;
| ~~~~~^~~~~~~~~~~
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [1, 2] into destination object 'keybuf' of size 8
659 | keybuf[2];
| ^~~~~~
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [2, 3] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [3, 4] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [4, 5] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [5, 6] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [6, 7] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [7, 8] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:675:22: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
675 | *q++ = *key << 1;
| ~~~~~^~~~~~~~~~~
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset 8 into destination object 'keybuf' of size 8
659 | keybuf[2];
| ^~~~~~
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [9, 10] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [10, 11] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [11, 12] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [12, 13] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [13, 14] into destination object 'keybuf' of size 8
../pgsql/contrib/pgcrypto/crypt-des.c:659:33: note: at offset [14, 15] into destination object 'keybuf' of size 8

It's possible to see by inspection that this loop doesn't actually
overrun the buffer. But the termination condition is oddly written:

q = (uint8 *) keybuf;
while (q - (uint8 *) keybuf - 8)

I think this odd coding, perhaps combined with all the cowboy casting
that's going on here, is what's confusing gcc into giving a warning.
I propose the attached patch to make this code less ugly and (with
luck) suppress the warning.

regards, tom lane