perf annotate: Check null of sym pointer before using it

Sym may be NULL, and that will cause perf to crash.
Signed-off-by: Shan Wei <davidshan@tencent.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/4FCD95D3.90209@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/ui/browsers/annotate.c

@@ -814,7 +814,7 @@ int symbol__tui_annotate(struct symbol *sym, struct map *map, int evidx,
 {
 	struct disasm_line *pos, *n;
 	struct annotation *notes;
-	const size_t size = symbol__size(sym);
+	size_t size;
 	struct map_symbol ms = {
 		.map = map,
 		.sym = sym,
@@ -834,6 +834,8 @@ int symbol__tui_annotate(struct symbol *sym, struct map *map, int evidx,
 	if (sym == NULL)
 		return -1;
 
+	size = symbol__size(sym);
+
 	if (map->dso->annotate_warned)
 		return -1;