[2.0] Fix bugs found by Coverity.

* **src/channels.cpp** -- `u` can never be `NULL` as the code higher up is setting it to the server user `ServerInstance->FakeClient` if it is `NULL`.

* **src/modules/m_joinflood.cpp** -- we are checking the value of these variables higher up and returning if they are less than 1.

* **src/users.cpp** -- `pos` can never be `std::string::npos` as this is a requirement of the loop.