perf symbols: Fix ppc64 SEGV in dso__load_sym with debuginfo files

64bit PowerPC debuginfo files have an empty function descriptor section.
I hit a SEGV when perf tried to use this section for symbol resolution.

To fix this we need to check the section is valid and we can do this by
checking for type SHT_PROGBITS.

Cc: <stable@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Eric B Munson <emunson@mgebm.net>
Link: http://lkml.kernel.org/r/20110824065242.895239970@samba.org


Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>