perf symbols: Stop using vmlinux files with no symbols

Fail if the kernel image contains no symbol, allowing using other images
in the vmlinux search path that may have a usable symtab.

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: 2nddept-manager@sdl.hitachi.co.jp
Cc: Francis Moreau <francis.moro@gmail.com>
Cc: Franck Bui-Huu <vagabon.xyz@gmail.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
LPU-Reference: <m3d3p9ydx9.fsf_-_@gmail.com>
Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -135,7 +135,7 @@ const char *kernel_get_module_path(const char *module)
 		if (dso__load_vmlinux(dso, map, vmlinux_name, NULL) <= 0)
 			return NULL;
 	} else {
-		if (dso__load_vmlinux_path(dso, map, NULL) < 0) {
+		if (dso__load_vmlinux_path(dso, map, NULL) <= 0) {
 			pr_debug("Failed to load kernel map.\n");
 			return NULL;
 		}