perf evlist: Fix alloc_mmap() failure path

perf evlist: Fix alloc_mmap() failure path

If zalloc fail, setting evlist->mmap[i].fd is unsafe and
perf_evlist__alloc_mmap() should bail out right after that.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Fixes: d4c6fb36ac2c ("perf evsel: Record fd into perf_mmap")
Link: http://lkml.kernel.org/r/1464699975-230440-1-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>