perf hists browser: Fix seg fault when annotate null symbol

In hists browser, press hotkey 'a' to annotate current symbol.

Now it causes segment fault if 'a' is pressed on a null symbol.

Here are 2 small bugs:
- In perf_evsel__hists_browse, the condition check after 'a' is pressed
  is not correct, we should check ->sym instead of ->map.
- In symbol__tui_annotate we must check whether sym is NULL or not
  before getting annotation structure.

This patch fixes above 2 small bugs.

Link: http://lkml.kernel.org/r/1302244286.4106.36.camel@minggr.sh.intel.com
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>