Fix bad memory accesses during srd_exit()

When the decoder and session unload functions are called they
remove themselves from the list. The code walking the list
must be careful to avoid accessing the next pointer which
might now be invalid. The g_slist_foreach() takes care of
this.

Reports from Valgrind before fix:

==175436== Invalid read of size 8
--
==175436==  Address 0xe3f2598 is 8 bytes inside a block of size 16 free'd
==175436==    at 0x4C2FDAC: free (vg_replace_malloc.c:530)
==175436==    by 0x563C541: g_free (in /usr/lib64/libglib-2.0.so.0.5600.3)
==175436==    by 0x5654783: g_slice_free1 (in /usr/lib64/libglib-2.0.so.0.5600.3)
==175436==    by 0x56552A2: g_slist_remove (in /usr/lib64/libglib-2.0.so.0.5600.3)
==175436==    by 0x4E3FEFF: srd_session_destroy (session.c:343)
==175436==    by 0x4E3F5C7: srd_exit (srd.c:311)
==175436==    by 0x40336F: test_inst_new (inst.c:40)
==175436==    by 0x53E51D5: srunner_run_tagged (in /usr/lib64/libcheck.so.0.0.0)
==175436==    by 0x401237: main (main.c:51)
==175436==  Block was alloc'd at
==175436==    at 0x4C2EBAB: malloc (vg_replace_malloc.c:299)
==175436==    by 0x563C435: g_malloc (in /usr/lib64/libglib-2.0.so.0.5600.3)
==175436==    by 0x5654056: g_slice_alloc (in /usr/lib64/libglib-2.0.so.0.5600.3)
==175436==    by 0x5655797: g_slist_append (in /usr/lib64/libglib-2.0.so.0.5600.3)
==175436==    by 0x4E3FC75: srd_session_new (session.c:71)
==175436==    by 0x403345: test_inst_new (inst.c:37)
==175436==    by 0x53E51D5: srunner_run_tagged (in /usr/lib64/libcheck.so.0.0.0)
==175436==    by 0x401237: main (main.c:51)

1 files changed, 9 insertions, 2 deletions

diff --git a/srd.c b/srd.c
index 535ea12..248d8e5 100644
--- a/srd.c
+++ b/srd.c
@@ -289,6 +289,12 @@ SRD_API int srd_init(const char *path)
 	return SRD_OK;
 }
 
+static void srd_session_destroy_cb(void *arg, void *ignored)
+{
+	(void)ignored; // Prevent unused warning
+	srd_session_destroy((struct srd_session *)arg);
+}
+
 /**
  * Shutdown libsigrokdecode.
  *
@@ -307,8 +313,9 @@ SRD_API int srd_exit(void)
 {
 	srd_dbg("Exiting libsigrokdecode.");
 
-	for (GSList *l = sessions; l; l = l->next)
-		srd_session_destroy(l->data);
+	g_slist_foreach(sessions, srd_session_destroy_cb, NULL);
+	g_slist_free(sessions);
+	sessions = NULL;
 
 	srd_decoder_unload_all();
 	g_slist_free_full(searchpaths, g_free);