perf annotate: Handle ins parsing failures

perf annotate: Handle ins parsing failures

Don't use the ins's ->sncprintf() if the parsing failed.

For example, this fixes the display of "imul %edx".  Without this patch:

       |      imul   (null),(null)

After this patch:

       |      imul   %edx

Signed-off-by: Rabin Vincent <rabin@rab.in>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1421607621-15005-1-git-send-email-rabin@rab.in
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>