MINOR: buffer: align the last output line of buffer_dump()

If the dumped length of buffer is not multiple of 16, the last output line can
be seen as below:

Dumping contents from byte 0 to byte 125
         0  1  2  3  4  5  6  7    8  9  a  b  c  d  e  f
  0000: 47 45 54 20 2f 69 6e 64 - 65 78 2e 68 74 6d 20 48   GET /index.htm H
  0010: 54 54 50 2f 31 2e 30 0d - 0a 55 73 65 72 2d 41 67   TTP/1.0..User-Ag
  ...
  0060: 30 0d 0a 43 6f 6e 6e 65 - 63 74 69 6f 6e 3a 20 4b   0..Connection: K
  0070: 65 65 70 2d 41 6c 69 76 - 65 0d 0a 0d 0a   eep-Alive....

Yes, the hex column will be overlapped by the text column. Both the hex and
text column should be aligned at their own area as below:

Dumping contents from byte 0 to byte 125
         0  1  2  3  4  5  6  7    8  9  a  b  c  d  e  f
  0000: 47 45 54 20 2f 69 6e 64 - 65 78 2e 68 74 6d 20 48   GET /index.htm H
  0010: 54 54 50 2f 31 2e 30 0d - 0a 55 73 65 72 2d 41 67   TTP/1.0..User-Ag
  ...
  0060: 30 0d 0a 43 6f 6e 6e 65 - 63 74 69 6f 6e 3a 20 4b   0..Connection: K
  0070: 65 65 70 2d 41 6c 69 76 - 65 0d 0a 0d 0a            eep-Alive....

Signed-off-by: Godbach <nylzhaowei@gmail.com>