[ticket/12922] Posts per page in MCP should have a min value of zero

In order to allow displaying the whole topic in MCP=>Main=>View topic,
the min value in the html input field must be 0.

PHPBB3-12922
[ticket/12922] Subsilver2 fix

PHPBB3-12922
Merge pull request #2816 from n-aleha/ticket/12922

[ticket/12922] Posts per page in MCP should have a min value of zero

* n-aleha/ticket/12922:
  [ticket/12922] Subsilver2 fix
  [ticket/12922] Posts per page in MCP should have a min value of zero