Improvements to bbp_validate_reply_to

[johnjamesjacoby]

The bbp_validate_reply_to() function frequently accepts values straight from user input, but without performing some of the more basic validations like is_numeric() or checking that the IDs are not negative numbers.

Given that "validation" is in the function name, it seems like an opportunity to make it more powerful and remove some duplication from where it is currently used.