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

In the Linux kernel, the following vulnerability has been resolved: mm: /proc/pid/smaps_rollup: fix no vma's null-deref Commit 258f669e7e88 ("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.