perf symbols: Fix crash in elf_section_by_name

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 <[email protected]>
Signed-off-by: Jiri Olsa <[email protected]>
Cc: Albert Strasheim <[email protected]>
Cc: Corey Ashford <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Paul Mackerras <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>