media: vimc: skip .s_stream() for stopped entities

In the Linux kernel, the following vulnerability has been resolved:

media: vimc: skip .s_stream() for stopped entities

Syzbot reported [1] a warning prompted by a check in call_s_stream()
that checks whether .s_stream() operation is warranted for unstarted
or stopped subdevs.

Add a simple fix in vimc_streamer_pipeline_terminate() ensuring that
entities skip a call to .s_stream() unless they have been previously
properly started.

[1] Syzbot report:
------------[ cut here ]------------
WARNING: CPU: 0 PID: 5933 at drivers/media/v4l2-core/v4l2-subdev.c:460 call_s_stream+0x2df/0x350 drivers/media/v4l2-core/v4l2-subdev.c:460
Modules linked in:
CPU: 0 UID: 0 PID: 5933 Comm: syz-executor330 Not tainted 6.13.0-rc2-syzkaller-00362-g2d8308bf5b67 #0
...
Call Trace:
<TASK>
vimc_streamer_pipeline_terminate+0x218/0x320 drivers/media/test-drivers/vimc/vimc-streamer.c:62
vimc_streamer_pipeline_init drivers/media/test-drivers/vimc/vimc-streamer.c:101 [inline]
vimc_streamer_s_stream+0x650/0x9a0 drivers/media/test-drivers/vimc/vimc-streamer.c:203
vimc_capture_start_streaming+0xa1/0x130 drivers/media/test-drivers/vimc/vimc-capture.c:256
vb2_start_streaming+0x15f/0x5a0 drivers/media/common/videobuf2/videobuf2-core.c:1789
vb2_core_streamon+0x2a7/0x450 drivers/media/common/videobuf2/videobuf2-core.c:2348
vb2_streamon drivers/media/common/videobuf2/videobuf2-v4l2.c:875 [inline]
vb2_ioctl_streamon+0xf4/0x170 drivers/media/common/videobuf2/videobuf2-v4l2.c:1118
__video_do_ioctl+0xaf0/0xf00 drivers/media/v4l2-core/v4l2-ioctl.c:3122
video_usercopy+0x4d2/0x1620 drivers/media/v4l2-core/v4l2-ioctl.c:3463
v4l2_ioctl+0x1ba/0x250 drivers/media/v4l2-core/v4l2-dev.c:366
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:906 [inline]
__se_sys_ioctl fs/ioctl.c:892 [inline]
__x64_sys_ioctl+0x190/0x200 fs/ioctl.c:892
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f2b85c01b19
...