perf trace: Fix off by one string allocation problem

perf trace: Fix off by one string allocation problem

We need to consider the null terminator, oops, fix it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Fixes: 017037ff3d0b ("perf trace: Allow specifying list of syscalls and events in -e/--expr/--event")
Link: http://lkml.kernel.org/n/tip-j79jpqqe91gvxqmsgxgfn2ni@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>