hush: source builtin should override $N only if it has args

function                                             old     new   delta
builtin_source                                       174     184     +10

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
ash: add comment about failures in source builtin. No code changes.

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
Merge git://busybox.net/busybox