Remove useless pointer update

buf is not used afterwards.  The compiler optimized the dead store out
anyway, but let's clean the source, too.
Signed-off-by: NRene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

tar-tree.c

@@ -94,7 +94,6 @@ static void write_blocked(void *buf, unsigned long size)
 	}
 	if (size) {
 		memcpy(block + offset, buf, size);
-		buf += size;
 		offset += size;
 	}
 	tail = offset % RECORDSIZE;