agent: Limit printed length of uname and password in debug output

agent: Limit printed length of uname and password in debug output

At this point in the code, neither the username nor password are
nul-terminated, so printing them in the debug output as nul-terminated
strings can easily result in rubbish being printed after them.

Fix this by specifying the string length in the printf() format string.
Finally my terminal will no longer ask me to install N’Ko fonts when
running the libnice tests.