block: Fix build with tracing enabled

filename was still uninitialised when it's used as a parameter to a
tracing function, so let's move the initialisation. Also, commit c2ad1b0c
forgot to add a NULL check, which this patch adds while we're at it.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Tested-by: NAndreas Färber <afaerber@suse.de>
Message-id: 1366645720-11384-1-git-send-email-kwolf@redhat.com
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

block.c

@@ -676,7 +676,13 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
     assert(bs->file == NULL);
     assert(options != NULL && bs->options != options);
 
-    trace_bdrv_open_common(bs, filename, flags, drv->format_name);
+    if (file != NULL) {
+        filename = file->filename;
+    } else {
+        filename = qdict_get_try_str(options, "filename");
+    }
+
+    trace_bdrv_open_common(bs, filename ?: "", flags, drv->format_name);
 
     if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv)) {
         return -ENOTSUP;
@@ -698,12 +704,6 @@ static int bdrv_open_common(BlockDriverState *bs, BlockDriverState *file,
         bdrv_enable_copy_on_read(bs);
     }
 
-    if (file != NULL) {
-        filename = file->filename;
-    } else {
-        filename = qdict_get_try_str(options, "filename");
-    }
-
     if (filename != NULL) {
         pstrcpy(bs->filename, sizeof(bs->filename), filename);
     } else {