Fix cleanup in arm_linux_software_single_step

Fix cleanup in arm_linux_software_single_step

I see the following error in testing aarch64 GDB debugging arm
program.

(gdb) PASS: gdb.reverse/readv-reverse.exp: set breakpoint at marker2
continue
Continuing.
=================================================================
==32273==ERROR: AddressSanitizer: attempting free on address which was not malloc()-ed: 0x000000ce4c00 in thread T0
    #0 0x2ba5615645c7 in __interceptor_free (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x545c7)^M
    #1 0x4be8b5 in VEC_CORE_ADDR_cleanup /home/yao/SourceCode/gnu/gdb/git/gdb/common/gdb_vecs.h:34^M
    #2 0x5e6d95 in do_my_cleanups /home/yao/SourceCode/gnu/gdb/git/gdb/common/cleanups.c:154^M
    #3 0x64c99a in fetch_inferior_event /home/yao/SourceCode/gnu/gdb/git/gdb/infrun.c:3975^M
    #4 0x678437 in inferior_event_handler /home/yao/SourceCode/gnu/gdb/git/gdb/inf-loop.c:44^M
    #5 0x5078f6 in remote_async_serial_handler /home/yao/SourceCode/gnu/gdb/git/gdb/remote.c:13223^M
    #6 0x4cecfd in run_async_handler_and_reschedule /home/yao/SourceCode/gnu/gdb/git/gdb/ser-base.c:137^M
    #7 0x676864 in gdb_wait_for_event /home/yao/SourceCode/gnu/gdb/git/gdb/event-loop.c:834^M
    #8 0x676a27 in gdb_do_one_event /home/yao/SourceCode/gnu/gdb/git/gdb/event-loop.c:323^M
    #9 0x676aed in start_event_loop /home/yao/SourceCode/gnu/gdb/git/gdb/event-loop.c:347^M
    #10 0x6706d2 in captured_command_loop /home/yao/SourceCode/gnu/gdb/git/gdb/main.c:318^M
    #11 0x66db8c in catch_errors /home/yao/SourceCode/gnu/gdb/git/gdb/exceptions.c:240^M
    #12 0x6716dd in captured_main /home/yao/SourceCode/gnu/gdb/git/gdb/main.c:1157^M
    #13 0x66db8c in catch_errors /home/yao/SourceCode/gnu/gdb/git/gdb/exceptions.c:240^M
    #14 0x671b7a in gdb_main /home/yao/SourceCode/gnu/gdb/git/gdb/main.c:1165^M
    #15 0x467684 in main /home/yao/SourceCode/gnu/gdb/git/gdb/gdb.c:32^M
    #16 0x2ba563ed7ec4 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21ec4)^M
    #17 0x4676b2 (/scratch/yao/gdb/build-git/aarch64-linux-gnu/gdb/gdb+0x4676b2)

looks we should discard cleanup if function
arm_linux_software_single_step returns early, or create cleanup when
it is needed.

gdb:

2016-02-16  Yao Qi  <yao.qi@linaro.org>

	* arm-linux-tdep.c (arm_linux_software_single_step): Assign
	'old_chain' later.