perf mmap: Be consistent when checking for an unmaped ring buffer

The previous patch is insufficient to cure the reported 'perf trace'
segfault, as it only cures the perf_mmap__read_done() case, moving the
segfault to perf_mmap__read_init() functio, fix it by doing the same
refcount check.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 8872481b ("perf mmap: Introduce perf_mmap__read_init()")
Link: https://lkml.kernel.org/r/20180326144127.GF18897@kernel.org


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