providers/virtualbox: check for provider config nilness

CHANGELOG.md

@@ -4,6 +4,10 @@ IMPROVEMENTS:
 
   - **New guest: Linux Mint** is now properly detected. [GH-3648]
 
+BUG FIXES:
+
+  - commands/package: `--base` works without crashing for VirtualBox.
+
 ## 1.6.0 (May 6, 2014)
 
 BACKWARDS INCOMPATIBILITIES:

plugins/providers/virtualbox/synced_folder.rb

@@ -6,6 +6,7 @@ module VagrantPlugins
       def usable?(machine, raise_errors=false)
         # These synced folders only work if the provider if VirtualBox
         machine.provider_name == :virtualbox &&
+          machine.provider_config &&
           machine.provider_config.functional_vboxsf
       end