disas: Always initialize read_memory_inner_func properly

I've recently seen this with valgrind while running the HMP tester:

==22373== Conditional jump or move depends on uninitialised value(s)
==22373==    at 0x4A41FD: arm_disas_set_info (cpu.c:504)
==22373==    by 0x3867A7: monitor_disas (disas.c:390)
==22373==    by 0x38E80E: memory_dump (monitor.c:1339)
==22373==    by 0x38FA43: handle_hmp_command (monitor.c:3123)
==22373==    by 0x38FB9E: qmp_human_monitor_command (monitor.c:613)
==22373==    by 0x4E3124: qmp_marshal_human_monitor_command (qmp-marshal.c:1736)
==22373==    by 0x769678: do_qmp_dispatch (qmp-dispatch.c:104)
==22373==    by 0x769678: qmp_dispatch (qmp-dispatch.c:131)
==22373==    by 0x38B734: handle_qmp_command (monitor.c:3853)
==22373==    by 0x76ED07: json_message_process_token (json-streamer.c:105)
==22373==    by 0x78D40A: json_lexer_feed_char (json-lexer.c:323)
==22373==    by 0x78D4CD: json_lexer_feed (json-lexer.c:373)
==22373==    by 0x38A08D: monitor_qmp_read (monitor.c:3895)

And indeed, in monitor_disas, the read_memory_inner_func variable was
not initialized, but arm_disas_set_info() expects this to be NULL
or a valid pointer. Let's properly set this to NULL in the
INIT_DISASSEMBLE_INFO to fix it in all functions that use the
disassemble_info struct.

Fixes: f7478a92dd9ee2276bfaa5b7317140d3f9d6a53b ("Fix Thumb-1 BE32 execution")
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1506524313-20037-1-git-send-email-thuth@redhat.com>
