daemon: free the string on stringsbuf add failure

If add_string_nodup fails free the passed string instead of leaking it,
as that string would have been owned by the stringbuf.

Adapt few places to this behaviour.

daemon/btrfs.c

@@ -1123,10 +1123,8 @@ do_btrfs_subvolume_show (const char *subvolume)
       }
 
       if (ss) {
-        if (add_string_nodup (&ret, ss) == -1) {
-          free (ss);
+        if (add_string_nodup (&ret, ss) == -1)
           return NULL;
-        }
       } else {
         if (add_string (&ret, "") == -1)
           return NULL;

daemon/devsparts.c

@@ -311,7 +311,6 @@ do_list_disk_labels (void)
 {
   DIR *dir = NULL;
   struct dirent *d;
-  char *rawdev = NULL;
   DECLARE_STRINGSBUF (ret);
 
   dir = opendir (GUESTFSDIR);
@@ -330,6 +329,7 @@ do_list_disk_labels (void)
   errno = 0;
   while ((d = readdir (dir)) != NULL) {
     CLEANUP_FREE char *path = NULL;
+    char *rawdev;
 
     if (d->d_name[0] == '.')
       continue;
@@ -347,12 +347,13 @@ do_list_disk_labels (void)
       goto error;
     }
 
-    if (add_string (&ret, d->d_name) == -1)
+    if (add_string (&ret, d->d_name) == -1) {
+      free (rawdev);
       goto error;
+    }
 
     if (add_string_nodup (&ret, rawdev) == -1)
       goto error;
-    rawdev = NULL;            /* buffer now owned by the stringsbuf */
   }
 
   /* Check readdir didn't fail */
@@ -380,6 +381,5 @@ do_list_disk_labels (void)
  error:
   if (dir)
     closedir (dir);
-  free (rawdev);
   return NULL;
 }

daemon/guestfsd.c

@@ -512,6 +512,7 @@ add_string_nodup (struct stringsbuf *sb, char *str)
       reply_with_perror ("realloc");
       free_stringslen (sb->argv, sb->size);
       sb->argv = NULL;
+      free (str);
       return -1;
     }
     sb->argv = new_argv;