Fix read beyond end of conditional argument on missing operand

nstr_parse_val() interprets argument "" as (empty) identifier, then
returns a pointer right beyond the end of the string.

The argument points into player->argbuf[].  If another argument
follows the conditional, it gets appended to the conditional.  Else,
whatever's left there from previous commands gets appended.  If the
argument is at the very end of player->argbuf[], we parse beyond the
buffer, until we run into a syntax error, or a zero byte.

Since player->argbuf[] is followed by a bunch of pointers, a syntax
error is almost certain.  If we somehow manage to parse all the
pointers and player->lasttime, the runaway parse will end at
player->btused, because that's definitely zero when conditionals get
parsed.