client/Client.cc: fix passing parameter to _lookup()

Fix cppcheck message:

[src/client/Client.cc:3903]: (performance) Passing the result
  of c_str() to a function that takes std::string as argument
  no. 2 is slow and redundant.

Instead using extra string variable and passing std::string::c_str()
to _lookup() use path[i].c_str().

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>