mm: /proc/pid/smaps_rollup: fix no vma's null-deref

stable inclusion
from stable-v4.19.264
commit dbe863bce7679c7f5ec0e993d834fe16c5e687b5
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I63UEU


CVE: NA

--------------------------------

Commit 258f669e ("mm: /proc/pid/smaps_rollup: convert to single value
seq_file") introduced a null-deref if there are no vma's in the task in
show_smaps_rollup.

Fixes: 258f669e ("mm: /proc/pid/smaps_rollup: convert to single value seq_file")
Signed-off-by: Seth Jenkins <sethjenkins@google.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Tested-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>