perf script: Avoid NULL dereference on symbol

perf script: Avoid NULL dereference on symbol

al->sym may be NULL given current if conditions and may cause a segv.

Fixes: d2bedb7863e9 ("perf script: Allow --symbol to accept hexadecimal addresses")
Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200421004329.43109-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>