gdb/python: fix invalid use disassemble_info::stream

After this commit:

  commit 81384924
  Date:   Tue Apr 5 11:06:16 2022 +0100

      gdb: have gdb_disassemble_info carry 'this' in its stream pointer

The disassemble_info::stream field will no longer be a ui_file*.  That
commit failed to update one location in py-disasm.c though.

While running some tests using the Python disassembler API, I
triggered a call to gdbpy_disassembler::print_address_func, and, as I
had compiled GDB with the undefined behaviour sanitizer, GDB crashed
as the code currently (incorrectly) casts the stream field to be a
ui_file*.

In this commit I fix this error.

In order to test this case I had to tweak the existing test case a
little.  I also spotted some debug printf statements in py-disasm.py,
which I have removed.