perf ui hist browser: Fix segfault on 'a' for annotate

There a typo in util/ui/browsers/hists.c that leads to a segfault when you
press the 'a' key on a non-resolved symbol (plain hex address).

LKML-Reference: <20100923201901.GE31726@gambetta>
Signed-off-by: Frederik Deweerdt <frederik.deweerdt@xprog.eu>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>