perf top: Fix annotate segv

Before we had sym_counter, it was initialized to zero and we used that
as an index in the global attrs variable, now we have a list of evsel
entries, and sym_counter became sym_evsel, that remained initialized to
zero (NULL): b00m.

Fix it by initializing it to the first entry in the evsel list.

Bug-introduced: 69aad6f1
Reported-by: Kirill Smelkov <kirr@mns.spb.ru>
Tested-by: Kirill Smelkov <kirr@mns.spb.ru>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Kirill Smelkov <kirr@mns.spb.ru>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>