Add pg_nodiscard decorations to base64 functions

The result of pg_b64_encode() and pg_b64_decode() should be checked
for errors.  This attribute could detect mistakes such as those fixed
in commit ff030ebe25 and d278541be4.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Ranier Vilela <ranier.vf@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CAEudQAq-3yHsSdWoOOaw%2BgAQYgPMpMGuB5pt2yCXgv-YuxG2Hg%40mail.gmail.com

src/include/common/base64.h

@@ -11,8 +11,8 @@
 #define BASE64_H
 
 /* base 64 */
-extern int	pg_b64_encode(const char *src, int len, char *dst, int dstlen);
-extern int	pg_b64_decode(const char *src, int len, char *dst, int dstlen);
+extern pg_nodiscard int pg_b64_encode(const char *src, int len, char *dst, int dstlen);
+extern pg_nodiscard int pg_b64_decode(const char *src, int len, char *dst, int dstlen);
 extern int	pg_b64_enc_len(int srclen);
 extern int	pg_b64_dec_len(int srclen);