ash: fix build failure when command built-in is disabled

ash: fix build failure when command built-in is disabled

Since commit 7eb8eecbb (ash: eval: Add assignment built-in support
again) building BusyBox with the 'command' built-in disabled fails.

parse_command_args() only needs to be called when the 'command'
built-in is run.  Which it won't be if it's disabled.

v2: Avoiding infinite loops is good, too.  Thanks, Harald van Dijk.

Reported-by: Deweloper <deweloper@wp.pl>
Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>