perf symbols: Fix crash in elf_section_by_name

Fixing crash in elf_section_by_name function caused by missing section
name in elf binary.

Reported-by: Albert Strasheim <albert@cloudflare.com>
Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Albert Strasheim <albert@cloudflare.com>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1393767127-599-1-git-send-email-jolsa@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>