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: NAlbert Strasheim <albert@cloudflare.com>
Signed-off-by: NJiri 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.comSigned-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/symbol-elf.c

@@ -151,15 +151,15 @@ Elf_Scn *elf_section_by_name(Elf *elf, GElf_Ehdr *ep,
 
 		gelf_getshdr(sec, shp);
 		str = elf_strptr(elf, ep->e_shstrndx, shp->sh_name);
-		if (!strcmp(name, str)) {
+		if (str && !strcmp(name, str)) {
 			if (idx)
 				*idx = cnt;
-			break;
+			return sec;
 		}
 		++cnt;
 	}
 
-	return sec;
+	return NULL;
 }
 
 #define elf_section__for_each_rel(reldata, pos, pos_mem, idx, nr_entries) \