perf tools: Check for null when copying nsinfo.

The argument to nsinfo__copy() was assumed to be valid, but some code paths
exist that will lead to NULL being passed.

In particular, running 'perf script -D' on a perf.data file containing an
PERF_RECORD_MMAP event associating the '[vdso]' dso with pid 0 earlier in
the event stream will lead to a segfault.

Since all calling code is already checking for a non-null return value,
just return NULL for this case as well.

Signed-off-by: Benno Evers <bevers@mesosphere.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180810133614.9925-1-bevers@mesosphere.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>