Fix fizzbuzz output length

fizzbuzz/fizzbuzz.s

@@ -110,9 +110,10 @@ save_to_buffer_loop:
         cmp $0, %rcx            # Have we processed all digits?
         jne save_to_buffer_loop # If not, loop again.
         # We're done moving data from the stack to the buffer.
-        # Let's add a newline to the end of our digits.
+        # Let's add a newline right after our digits.
         # The ASCII integer representation of "\n" is 10.
-        movb $10, number_string + 3
+        # %rdx is left pointing at the newline, so callers can compute length.
+        movb $10, (%rdx)
         mov %rbp, %rsp
         pop %rbp
         ret
@@ -159,9 +160,10 @@ print_counter:
         mov $4, %rax
         mov $1, %rbx
         mov $number_string, %rcx # Set the buffer the kernel should use.
-        # Print 3 digits + 1 newline = 4 bytes.
-	# FIX: this is wrong -- we're not always printing 4 bytes.
-        mov $4, %rdx
+        # _itoa leaves %rdx pointing at the newline after the digits.
+        # Length = (newline address - buffer start) + 1 for the newline.
+        sub $number_string, %rdx
+        inc %rdx
         jmp print_and_return
 print_fizz:
         mov $fizz, %rcx