perf srcline: Do not consider empty files as valid srclines

Sometimes we get a non-null, but empty, string for the filename from
bfd. This then results in srclines of the form ":0", which is different
from the canonical SRCLINE_UNKNOWN in the form "??:0".  Set the file to
NULL if it is empty to fix this.

Signed-off-by: Milian Wolff <milian.wolff@kdab.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Yao Jin <yao.jin@linux.intel.com>
Link: http://lkml.kernel.org/r/20170806212446.24925-14-milian.wolff@kdab.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>