[gdb/tui] Make tui_disassemble more efficient

Function tui_disassemble (with addr_size parameter) has two modes of
operation:
- addr_size != nullptr, and
- addr_size == nullptr.

I noticed that for the addr_size == nullptr case, more than necessary is done.

Fix this by using continue and null_stream.

While we're at it, eliminate the unnecessary variables new_size and orig_pc.

Tested on x86_64-linux.

Approved-By: Andrew Burgess <aburgess@redhat.com>