freerdp/core/gcc: channel name hardening

freerdp/core/gcc: channel name hardening

According to [MS-RDPBCGR 2.2.1.3.4.1 Channel Definition Structure]
the channel name must be an 8-byte array containing a null-terminated
collection of seven ANSI characters that uniquely identify the channel.

We did not check if the transmitted name was null-terminated which
could have the usual severe effects on stabiliy and security since
the channel name is used in several functions expecting a null-
terminated string (strlen, printf, etc.)