Initialize login and password to avoid warning or worse.

Initialize char* login and password in try_security_mechanism() to avoid
error when building with current llvm clang compiler.

clang bails with "error: variable 'login' is used uninitialized whenever
'&&' condition is false [-Werror,-Wsometimes-uninitialized]", and then a
similar message for 'password'.

If the streq() condition is false, login and password may be pointed
who-knows-where when free()'d later.