gdb/record: add an assert in cmd_record_start

The 'record' command is both a prefix command AND an alias for 'target
record-full'.  As it is a prefix command, if a user types:

  (gdb) record blah

Then GDB will look for, and try to invoke the 'blah' sub-command.
This will either succeed (if blah is found) or throw an error (if blah
is not found).

As such, the only way to invoke the 'record' command is like:

  (gdb) record

It is impossible to pass arguments to the 'record' command.  As we
know this is true then we can assert this in cmd_record_start.

I added this assert because initially I was going to try forwarding
ARGS from cmd_record_start to the 'target record-full' command, but
then I realised passing arguments to 'record' was impossible.

There should be no user visible changes after this commit.

Approved-By: Tom Tromey <tom@tromey.com>