Fix player_login() not to print null pointer

Fix player_login() not to print null pointer

When the input line contains only whitespace, player->argp[0] is null
after parse().  This condition was not checked, comtch() returned
M_IGNORE, and code handling that printed the null pointer.  Some
systems (GNU, Windows) deal gracefully with printing null strings,
others crash.  Trivial to trigger remotely.

Before the fix, the value of parse() was assigned, but never used, and
that was spotted by the Clang Static Analyzer.