disk.c ensure a valid volume is returned from get_free_volume()

volume is used for element math and indexing into buffers
-1  volume is bad

Change-Id: I1ec75ef3a848c154ea80905224ea9a3a14bf8151

firmware/common/disk.c

@@ -174,6 +174,13 @@ int disk_mount(int drive)
 
     int volume = get_free_volume();
 
+    if (volume < 0)
+    {
+        DEBUGF("No Free Volumes\n");
+        disk_writer_unlock();
+        return 0;
+    }
+
     if (!disk_init(IF_MD(drive)))
     {
         disk_writer_unlock();