Don't crash when compile of CodeWScope fails

code_with_scope_compile.diff

diff --git a/scripting/engine_spidermonkey.cpp b/scripting/engine_spidermonkey.cpp
index 88f48e3..0f36817 100644
--- a/scripting/engine_spidermonkey.cpp
+++ b/scripting/engine_spidermonkey.cpp
@@ -662,6 +662,9 @@ namespace mongo {
             case CodeWScope: {
                 JSFunction * func = compileFunction( e.codeWScopeCode() );
 
+                if ( !func ) 
+                  return JSVAL_NULL;
+                
                 BSONObj extraScope = e.codeWScopeObject();
                 if ( ! extraScope.isEmpty() ) {
                     log() << "warning: CodeWScope doesn't transfer to db.eval" << endl;