fix boost::network::uri::decode error - out of range because of '%'

[cnagune]

problem

• if template type is "const char *" in boost::network::uri::detail::decode function,
  segmentation fault can occor.

cause

• "const char *" type is not throw std::out_of_range

fix

• check out of range

[deanberris]

LGTM -- good catch! Thanks @cnagune!