Really fix the dummy implementations in cipher.c.

945083b2 wasn't enough to silence compiler warnings.

src/common/cipher.c

@@ -21,10 +21,12 @@
 
 static void cipher_failure(void) pg_attribute_noreturn();
 
+
 PgCipherCtx *
 pg_cipher_ctx_create(int cipher, uint8 *key, int klen, bool enc)
 {
 	cipher_failure();
+	return NULL;				/* keep compiler quiet */
 }
 
 void
@@ -40,6 +42,7 @@ pg_cipher_encrypt(PgCipherCtx *ctx, const unsigned char *plaintext,
 				  unsigned char *outtag, const int taglen)
 {
 	cipher_failure();
+	return false;				/* keep compiler quiet */
 }
 
 bool
@@ -49,6 +52,7 @@ pg_cipher_decrypt(PgCipherCtx *ctx, const unsigned char *ciphertext,
 				  unsigned char *intag, const int taglen)
 {
 	cipher_failure();
+	return false;				/* keep compiler quiet */
 }
 
 static void
@@ -64,4 +68,3 @@ cipher_failure(void)
 	exit(1);
 #endif
 }
-