perf evlist: Check 'base' pointer before checking refcnt when put a mmap

evlist->mmap[i]->refcnt could be 0 if an evlist has no evsel or if all
evsels don't match the evlist during mmap. For example, when all evsels
are overwritable but the evlist itself is normal. To avoid crashing,
perf should check 'base' pointer before checking refcnt, and raise bug
only when base is not NULL.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1464183898-174512-2-git-send-email-wangnan0@huawei.com


[ Renamed 'mmap' variable, it is reserved in old distros such as Ubuntu 12.04, breaking the build ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>