perf symbols: Don't try to synthesize plt without dynstr

If .dynsym exists but .dynstr is empty (NO_BITS or size==0), a segfault
occurs.  Avoid this by checking that .dynstr is not empty.
Signed-off-by: Cody P Schafer <cody@linux.vnet.ibm.com>
Cc: David Hansen <dave@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Matt Hellsley <matthltc@us.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/1344637382-22789-6-git-send-email-cody@linux.vnet.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/symbol-elf.c

@@ -232,6 +232,9 @@ int dso__synthesize_plt_symbols(struct dso *dso, char *name, struct map *map,
 	if (symstrs == NULL)
 		goto out_elf_end;
 
+	if (symstrs->d_size == 0)
+		goto out_elf_end;
+
 	nr_rel_entries = shdr_rel_plt.sh_size / shdr_rel_plt.sh_entsize;
 	plt_offset = shdr_plt.sh_offset;