Fix issue with the import message commands introduced in previous commit

The fread() usage here did not need "addressing" (Commit 1b56dc96).

This caused messages posted via smbutil to be short (e.g. 0 or 1 bytes
in length). Reported by Accession via Idle Relay Chat.

Also, don't call strlen() on msgtxt which could be NULL, causing segfault
in final printf() callin in postmsg().