argon2: Avoid cppcheck integer oveflow warning

This patch avoid warning in constant-time base64 function:

Error: CPPCHECK_WARNING (CWE-190): [#def2]
cryptsetup-main/lib/crypto_backend/argon2/encoding.c:86: error[integerOverflow]: Signed integer overflow for expression ''0'-52'.

The (x + ('0' - 52)) can be rewritten to (x - (52 - '0')) with the same effect.
Similar solution used in https://github.com/pornin/CTTK/blob/master/src/base64.c

lib/crypto_backend/argon2/encoding.c

@@ -83,7 +83,7 @@
 static int b64_byte_to_char(unsigned x) {
     return (LT(x, 26) & (x + 'A')) |
            (GE(x, 26) & LT(x, 52) & (x + ('a' - 26))) |
-           (GE(x, 52) & LT(x, 62) & (x + ('0' - 52))) | (EQ(x, 62) & '+') |
+           (GE(x, 52) & LT(x, 62) & (x - (52 - '0'))) | (EQ(x, 62) & '+') |
            (EQ(x, 63) & '/');
 }