SERVER-12380 Fix error that prevented UpdateDriver from being reused.

The UpdateDriver represents a document->document transformation described by the
update parameter of update-like database operations.  It could already be used
repeatedly on multiple documents, so long as it represented only a single transaction.
However, it was designed to also support reprogramming to handle different transformations.
A programming error prevented this, by not clearing a piece of old state when reparsing.

src/mongo/db/ops/update_driver.cpp

@@ -439,6 +439,7 @@ namespace mongo {
         for (vector<ModifierInterface*>::iterator it = _mods.begin(); it != _mods.end(); ++it) {
             delete *it;
         }
+        _mods.clear();
         _indexedFields = NULL;
         _replacementMode = false;
         _positional = false;