Get rid of unclever assert() in Send_Message_Mask()

Get rid of unclever assert() in Send_Message_Mask()

Either we use assert() to _guarantee_ a certain condition, or we use
if(...) to test for it. But never both.

So get rid of the assert() in Send_Message_Mask() and handle the case
that the target mask doesn't contain a dot (".") as regular error,
don't require the caller to assure that any more.

This polishes commit 5a312824.

Please note:
The test in Send_Message() is still _required_ to detect whether the
target is a channel (no dot) or a "target mask" (at least one dot)!