perf probe: Fix a segfault bug in debuginfo_cache

perf probe --list will get a segfault if the first kprobe event is on a
module and the second or latter one is on the kernel.

e.g.
  ----
  # ./perf probe -q -m pcspkr pcspkr_event
  # ./perf probe -q vfs_read
  # ./perf probe -l
  Segmentation fault (core dumped)
  ----

This is because the debuginfo_cache fails to handle NULL module name,
which causes segfault on strcmp. (Note that strcmp("something", NULL)
always causes segfault)

To fix this debuginfo_cache__open always translates the NULL module name
to "kernel" (this is correct, because NULL module name means opening the
debuginfo for the kernel)

  ----
  # ./perf probe -l
    probe:pcspkr_event   (on pcspkr_event@drivers/input/misc/pcspkr.c
    in pcspkr)
    probe:vfs_read       (on vfs_read@ksrc/linux-3/fs/read_write.c)
  ----
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20150930164135.3733.23993.stgit@localhost.localdomainSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -441,20 +441,23 @@ static char *debuginfo_cache_path;
 
 static struct debuginfo *debuginfo_cache__open(const char *module, bool silent)
 {
-	if ((debuginfo_cache_path && !strcmp(debuginfo_cache_path, module)) ||
-	    (!debuginfo_cache_path && !module && debuginfo_cache))
+	const char *path = module;
+
+	/* If the module is NULL, it should be the kernel. */
+	if (!module)
+		path = "kernel";
+
+	if (debuginfo_cache_path && !strcmp(debuginfo_cache_path, path))
 		goto out;
 
 	/* Copy module path */
 	free(debuginfo_cache_path);
-	if (module) {
-		debuginfo_cache_path = strdup(module);
+	debuginfo_cache_path = strdup(path);
 	if (!debuginfo_cache_path) {
 		debuginfo__delete(debuginfo_cache);
 		debuginfo_cache = NULL;
 		goto out;
 	}
-	}
 
 	debuginfo_cache = open_debuginfo(module, silent);
 	if (!debuginfo_cache)