perf buildid: Do not update SDT cache with null filename

perf buildid: Do not update SDT cache with null filename

Valgrind was complaining:

  ==2633== Syscall param open(filename) points to unaddressable byte(s)
  ==2633==    at 0x5281CC0: __open_nocancel (syscall-template.S:84)
  ==2633==    by 0x537D38: open (fcntl2.h:53)
  ==2633==    by 0x537D38: get_sdt_note_list (symbol-elf.c:2017)
  ==2633==    by 0x5396FD: probe_cache__scan_sdt (probe-file.c:700)
  ==2633==    by 0x49EA2C: build_id_cache__add_sdt_cache (build-id.c:625)
  ==2633==    by 0x49EA2C: build_id_cache__add_s (build-id.c:697)
  ==2633==    by 0x49EE72: build_id_cache__add_b (build-id.c:717)
  ==2633==    by 0x49EE72: dso__cache_build_id (build-id.c:782)
  ==2633==    by 0x49F190: __dsos__cache_build_ids (build-id.c:793)
  ==2633==    by 0x49F190: machine__cache_build_ids (build-id.c:801)
  ==2633==    by 0x49F190: perf_session__cache_build_ids (build-id.c:815)
  ==2633==    by 0x4CD4F2: write_build_id (header.c:165)
  ==2633==    by 0x4D26F7: do_write_feat (header.c:2296)
  ==2633==    by 0x4D26F7: perf_header__adds_write (header.c:2335)
  ==2633==    by 0x4D26F7: perf_session__write_header (header.c:2414)
  ==2633==    by 0x43B324: __cmd_record (builtin-record.c:1154)
  ==2633==    by 0x43B324: cmd_record (builtin-record.c:1839)
  ==2633==    by 0x455A07: __cmd_record (builtin-kmem.c:1868)
  ==2633==    by 0x455A07: cmd_kmem (builtin-kmem.c:1944)
  ==2633==    by 0x497150: run_builtin (perf.c:359)
  ==2633==    by 0x428CE0: handle_internal_command (perf.c:421)
  ==2633==    by 0x428CE0: run_argv (perf.c:467)
  ==2633==    by 0x428CE0: main (perf.c:614)
  ==2633==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tommi Rantala <tommi.t.rantala@nokia.com>
Link: http://lkml.kernel.org/r/20170322130624.21881-2-tommi.t.rantala@nokia.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>