option parsing crash: 'playmode' without value

option parsing crash: 'playmode' without value

Reported directly to devteam, specifying the playmode option without
appending a value caused a segfault during options parsing.  The
argument passed to string_for_opt() ought to have been False for
"not optional" to trigger a "bad syntax" message, but even then the
playmode parsing needs to check for Null to prevent the crash.

"Bad syntax" is awfully vague.  In cases like this, we can be more
precise.  Now it will say "Missing value" instead.